287 lines
12 KiB
Plaintext
287 lines
12 KiB
Plaintext
-*-Mode: outline-*-
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Light-weight System Calls for IA-64
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-----------------------------------
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Started: 13-Jan-2003
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Last update: 27-Sep-2003
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David Mosberger-Tang
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<davidm@hpl.hp.com>
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Using the "epc" instruction effectively introduces a new mode of
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execution to the ia64 linux kernel. We call this mode the
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"fsys-mode". To recap, the normal states of execution are:
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- kernel mode:
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Both the register stack and the memory stack have been
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switched over to kernel memory. The user-level state is saved
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in a pt-regs structure at the top of the kernel memory stack.
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- user mode:
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Both the register stack and the kernel stack are in
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user memory. The user-level state is contained in the
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CPU registers.
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- bank 0 interruption-handling mode:
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This is the non-interruptible state which all
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interruption-handlers start execution in. The user-level
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state remains in the CPU registers and some kernel state may
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be stored in bank 0 of registers r16-r31.
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In contrast, fsys-mode has the following special properties:
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- execution is at privilege level 0 (most-privileged)
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- CPU registers may contain a mixture of user-level and kernel-level
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state (it is the responsibility of the kernel to ensure that no
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security-sensitive kernel-level state is leaked back to
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user-level)
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- execution is interruptible and preemptible (an fsys-mode handler
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can disable interrupts and avoid all other interruption-sources
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to avoid preemption)
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- neither the memory-stack nor the register-stack can be trusted while
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in fsys-mode (they point to the user-level stacks, which may
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be invalid, or completely bogus addresses)
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In summary, fsys-mode is much more similar to running in user-mode
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than it is to running in kernel-mode. Of course, given that the
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privilege level is at level 0, this means that fsys-mode requires some
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care (see below).
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* How to tell fsys-mode
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Linux operates in fsys-mode when (a) the privilege level is 0 (most
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privileged) and (b) the stacks have NOT been switched to kernel memory
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yet. For convenience, the header file <asm-ia64/ptrace.h> provides
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three macros:
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user_mode(regs)
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user_stack(task,regs)
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fsys_mode(task,regs)
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The "regs" argument is a pointer to a pt_regs structure. The "task"
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argument is a pointer to the task structure to which the "regs"
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pointer belongs to. user_mode() returns TRUE if the CPU state pointed
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to by "regs" was executing in user mode (privilege level 3).
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user_stack() returns TRUE if the state pointed to by "regs" was
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executing on the user-level stack(s). Finally, fsys_mode() returns
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TRUE if the CPU state pointed to by "regs" was executing in fsys-mode.
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The fsys_mode() macro is equivalent to the expression:
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!user_mode(regs) && user_stack(task,regs)
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* How to write an fsyscall handler
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The file arch/ia64/kernel/fsys.S contains a table of fsyscall-handlers
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(fsyscall_table). This table contains one entry for each system call.
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By default, a system call is handled by fsys_fallback_syscall(). This
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routine takes care of entering (full) kernel mode and calling the
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normal Linux system call handler. For performance-critical system
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calls, it is possible to write a hand-tuned fsyscall_handler. For
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example, fsys.S contains fsys_getpid(), which is a hand-tuned version
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of the getpid() system call.
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The entry and exit-state of an fsyscall handler is as follows:
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** Machine state on entry to fsyscall handler:
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- r10 = 0
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- r11 = saved ar.pfs (a user-level value)
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- r15 = system call number
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- r16 = "current" task pointer (in normal kernel-mode, this is in r13)
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- r32-r39 = system call arguments
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- b6 = return address (a user-level value)
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- ar.pfs = previous frame-state (a user-level value)
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- PSR.be = cleared to zero (i.e., little-endian byte order is in effect)
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- all other registers may contain values passed in from user-mode
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** Required machine state on exit to fsyscall handler:
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- r11 = saved ar.pfs (as passed into the fsyscall handler)
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- r15 = system call number (as passed into the fsyscall handler)
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- r32-r39 = system call arguments (as passed into the fsyscall handler)
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- b6 = return address (as passed into the fsyscall handler)
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- ar.pfs = previous frame-state (as passed into the fsyscall handler)
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Fsyscall handlers can execute with very little overhead, but with that
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speed comes a set of restrictions:
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o Fsyscall-handlers MUST check for any pending work in the flags
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member of the thread-info structure and if any of the
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TIF_ALLWORK_MASK flags are set, the handler needs to fall back on
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doing a full system call (by calling fsys_fallback_syscall).
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o Fsyscall-handlers MUST preserve incoming arguments (r32-r39, r11,
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r15, b6, and ar.pfs) because they will be needed in case of a
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system call restart. Of course, all "preserved" registers also
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must be preserved, in accordance to the normal calling conventions.
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o Fsyscall-handlers MUST check argument registers for containing a
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NaT value before using them in any way that could trigger a
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NaT-consumption fault. If a system call argument is found to
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contain a NaT value, an fsyscall-handler may return immediately
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with r8=EINVAL, r10=-1.
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o Fsyscall-handlers MUST NOT use the "alloc" instruction or perform
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any other operation that would trigger mandatory RSE
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(register-stack engine) traffic.
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o Fsyscall-handlers MUST NOT write to any stacked registers because
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it is not safe to assume that user-level called a handler with the
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proper number of arguments.
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o Fsyscall-handlers need to be careful when accessing per-CPU variables:
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unless proper safe-guards are taken (e.g., interruptions are avoided),
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execution may be pre-empted and resumed on another CPU at any given
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time.
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o Fsyscall-handlers must be careful not to leak sensitive kernel'
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information back to user-level. In particular, before returning to
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user-level, care needs to be taken to clear any scratch registers
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that could contain sensitive information (note that the current
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task pointer is not considered sensitive: it's already exposed
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through ar.k6).
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o Fsyscall-handlers MUST NOT access user-memory without first
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validating access-permission (this can be done typically via
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probe.r.fault and/or probe.w.fault) and without guarding against
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memory access exceptions (this can be done with the EX() macros
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defined by asmmacro.h).
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The above restrictions may seem draconian, but remember that it's
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possible to trade off some of the restrictions by paying a slightly
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higher overhead. For example, if an fsyscall-handler could benefit
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from the shadow register bank, it could temporarily disable PSR.i and
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PSR.ic, switch to bank 0 (bsw.0) and then use the shadow registers as
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needed. In other words, following the above rules yields extremely
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fast system call execution (while fully preserving system call
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semantics), but there is also a lot of flexibility in handling more
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complicated cases.
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* Signal handling
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The delivery of (asynchronous) signals must be delayed until fsys-mode
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is exited. This is accomplished with the help of the lower-privilege
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transfer trap: arch/ia64/kernel/process.c:do_notify_resume_user()
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checks whether the interrupted task was in fsys-mode and, if so, sets
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PSR.lp and returns immediately. When fsys-mode is exited via the
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"br.ret" instruction that lowers the privilege level, a trap will
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occur. The trap handler clears PSR.lp again and returns immediately.
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The kernel exit path then checks for and delivers any pending signals.
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* PSR Handling
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The "epc" instruction doesn't change the contents of PSR at all. This
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is in contrast to a regular interruption, which clears almost all
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bits. Because of that, some care needs to be taken to ensure things
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work as expected. The following discussion describes how each PSR bit
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is handled.
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PSR.be Cleared when entering fsys-mode. A srlz.d instruction is used
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to ensure the CPU is in little-endian mode before the first
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load/store instruction is executed. PSR.be is normally NOT
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restored upon return from an fsys-mode handler. In other
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words, user-level code must not rely on PSR.be being preserved
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across a system call.
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PSR.up Unchanged.
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PSR.ac Unchanged.
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PSR.mfl Unchanged. Note: fsys-mode handlers must not write-registers!
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PSR.mfh Unchanged. Note: fsys-mode handlers must not write-registers!
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PSR.ic Unchanged. Note: fsys-mode handlers can clear the bit, if needed.
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PSR.i Unchanged. Note: fsys-mode handlers can clear the bit, if needed.
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PSR.pk Unchanged.
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PSR.dt Unchanged.
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PSR.dfl Unchanged. Note: fsys-mode handlers must not write-registers!
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PSR.dfh Unchanged. Note: fsys-mode handlers must not write-registers!
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PSR.sp Unchanged.
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PSR.pp Unchanged.
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PSR.di Unchanged.
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PSR.si Unchanged.
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PSR.db Unchanged. The kernel prevents user-level from setting a hardware
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breakpoint that triggers at any privilege level other than 3 (user-mode).
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PSR.lp Unchanged.
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PSR.tb Lazy redirect. If a taken-branch trap occurs while in
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fsys-mode, the trap-handler modifies the saved machine state
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such that execution resumes in the gate page at
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syscall_via_break(), with privilege level 3. Note: the
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taken branch would occur on the branch invoking the
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fsyscall-handler, at which point, by definition, a syscall
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restart is still safe. If the system call number is invalid,
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the fsys-mode handler will return directly to user-level. This
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return will trigger a taken-branch trap, but since the trap is
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taken _after_ restoring the privilege level, the CPU has already
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left fsys-mode, so no special treatment is needed.
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PSR.rt Unchanged.
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PSR.cpl Cleared to 0.
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PSR.is Unchanged (guaranteed to be 0 on entry to the gate page).
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PSR.mc Unchanged.
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PSR.it Unchanged (guaranteed to be 1).
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PSR.id Unchanged. Note: the ia64 linux kernel never sets this bit.
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PSR.da Unchanged. Note: the ia64 linux kernel never sets this bit.
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PSR.dd Unchanged. Note: the ia64 linux kernel never sets this bit.
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PSR.ss Lazy redirect. If set, "epc" will cause a Single Step Trap to
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be taken. The trap handler then modifies the saved machine
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state such that execution resumes in the gate page at
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syscall_via_break(), with privilege level 3.
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PSR.ri Unchanged.
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PSR.ed Unchanged. Note: This bit could only have an effect if an fsys-mode
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handler performed a speculative load that gets NaTted. If so, this
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would be the normal & expected behavior, so no special treatment is
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needed.
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PSR.bn Unchanged. Note: fsys-mode handlers may clear the bit, if needed.
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Doing so requires clearing PSR.i and PSR.ic as well.
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PSR.ia Unchanged. Note: the ia64 linux kernel never sets this bit.
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* Using fast system calls
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To use fast system calls, userspace applications need simply call
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__kernel_syscall_via_epc(). For example
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-- example fgettimeofday() call --
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-- fgettimeofday.S --
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#include <asm/asmmacro.h>
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GLOBAL_ENTRY(fgettimeofday)
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.prologue
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.save ar.pfs, r11
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mov r11 = ar.pfs
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.body
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mov r2 = 0xa000000000020660;; // gate address
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// found by inspection of System.map for the
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// __kernel_syscall_via_epc() function. See
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// below for how to do this for real.
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mov b7 = r2
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mov r15 = 1087 // gettimeofday syscall
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;;
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br.call.sptk.many b6 = b7
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;;
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.restore sp
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mov ar.pfs = r11
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br.ret.sptk.many rp;; // return to caller
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END(fgettimeofday)
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-- end fgettimeofday.S --
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In reality, getting the gate address is accomplished by two extra
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values passed via the ELF auxiliary vector (include/asm-ia64/elf.h)
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o AT_SYSINFO : is the address of __kernel_syscall_via_epc()
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o AT_SYSINFO_EHDR : is the address of the kernel gate ELF DSO
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The ELF DSO is a pre-linked library that is mapped in by the kernel at
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the gate page. It is a proper ELF shared object so, with a dynamic
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loader that recognises the library, you should be able to make calls to
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the exported functions within it as with any other shared library.
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AT_SYSINFO points into the kernel DSO at the
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__kernel_syscall_via_epc() function for historical reasons (it was
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used before the kernel DSO) and as a convenience.
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