301 lines
13 KiB
ReStructuredText
301 lines
13 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
=================
|
|
KVM Lock Overview
|
|
=================
|
|
|
|
1. Acquisition Orders
|
|
---------------------
|
|
|
|
The acquisition orders for mutexes are as follows:
|
|
|
|
- cpus_read_lock() is taken outside kvm_lock
|
|
|
|
- kvm->lock is taken outside vcpu->mutex
|
|
|
|
- kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock
|
|
|
|
- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
|
|
them together is quite rare.
|
|
|
|
- kvm->mn_active_invalidate_count ensures that pairs of
|
|
invalidate_range_start() and invalidate_range_end() callbacks
|
|
use the same memslots array. kvm->slots_lock and kvm->slots_arch_lock
|
|
are taken on the waiting side when modifying memslots, so MMU notifiers
|
|
must not take either kvm->slots_lock or kvm->slots_arch_lock.
|
|
|
|
For SRCU:
|
|
|
|
- ``synchronize_srcu(&kvm->srcu)`` is called inside critical sections
|
|
for kvm->lock, vcpu->mutex and kvm->slots_lock. These locks _cannot_
|
|
be taken inside a kvm->srcu read-side critical section; that is, the
|
|
following is broken::
|
|
|
|
srcu_read_lock(&kvm->srcu);
|
|
mutex_lock(&kvm->slots_lock);
|
|
|
|
- kvm->slots_arch_lock instead is released before the call to
|
|
``synchronize_srcu()``. It _can_ therefore be taken inside a
|
|
kvm->srcu read-side critical section, for example while processing
|
|
a vmexit.
|
|
|
|
On x86:
|
|
|
|
- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock and kvm->arch.xen.xen_lock
|
|
|
|
- kvm->arch.mmu_lock is an rwlock; critical sections for
|
|
kvm->arch.tdp_mmu_pages_lock and kvm->arch.mmu_unsync_pages_lock must
|
|
also take kvm->arch.mmu_lock
|
|
|
|
Everything else is a leaf: no other lock is taken inside the critical
|
|
sections.
|
|
|
|
2. Exception
|
|
------------
|
|
|
|
Fast page fault:
|
|
|
|
Fast page fault is the fast path which fixes the guest page fault out of
|
|
the mmu-lock on x86. Currently, the page fault can be fast in one of the
|
|
following two cases:
|
|
|
|
1. Access Tracking: The SPTE is not present, but it is marked for access
|
|
tracking. That means we need to restore the saved R/X bits. This is
|
|
described in more detail later below.
|
|
|
|
2. Write-Protection: The SPTE is present and the fault is caused by
|
|
write-protect. That means we just need to change the W bit of the spte.
|
|
|
|
What we use to avoid all the races is the Host-writable bit and MMU-writable bit
|
|
on the spte:
|
|
|
|
- Host-writable means the gfn is writable in the host kernel page tables and in
|
|
its KVM memslot.
|
|
- MMU-writable means the gfn is writable in the guest's mmu and it is not
|
|
write-protected by shadow page write-protection.
|
|
|
|
On fast page fault path, we will use cmpxchg to atomically set the spte W
|
|
bit if spte.HOST_WRITEABLE = 1 and spte.WRITE_PROTECT = 1, to restore the saved
|
|
R/X bits if for an access-traced spte, or both. This is safe because whenever
|
|
changing these bits can be detected by cmpxchg.
|
|
|
|
But we need carefully check these cases:
|
|
|
|
1) The mapping from gfn to pfn
|
|
|
|
The mapping from gfn to pfn may be changed since we can only ensure the pfn
|
|
is not changed during cmpxchg. This is a ABA problem, for example, below case
|
|
will happen:
|
|
|
|
+------------------------------------------------------------------------+
|
|
| At the beginning:: |
|
|
| |
|
|
| gpte = gfn1 |
|
|
| gfn1 is mapped to pfn1 on host |
|
|
| spte is the shadow page table entry corresponding with gpte and |
|
|
| spte = pfn1 |
|
|
+------------------------------------------------------------------------+
|
|
| On fast page fault path: |
|
|
+------------------------------------+-----------------------------------+
|
|
| CPU 0: | CPU 1: |
|
|
+------------------------------------+-----------------------------------+
|
|
| :: | |
|
|
| | |
|
|
| old_spte = *spte; | |
|
|
+------------------------------------+-----------------------------------+
|
|
| | pfn1 is swapped out:: |
|
|
| | |
|
|
| | spte = 0; |
|
|
| | |
|
|
| | pfn1 is re-alloced for gfn2. |
|
|
| | |
|
|
| | gpte is changed to point to |
|
|
| | gfn2 by the guest:: |
|
|
| | |
|
|
| | spte = pfn1; |
|
|
+------------------------------------+-----------------------------------+
|
|
| :: |
|
|
| |
|
|
| if (cmpxchg(spte, old_spte, old_spte+W) |
|
|
| mark_page_dirty(vcpu->kvm, gfn1) |
|
|
| OOPS!!! |
|
|
+------------------------------------------------------------------------+
|
|
|
|
We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
|
|
|
|
For direct sp, we can easily avoid it since the spte of direct sp is fixed
|
|
to gfn. For indirect sp, we disabled fast page fault for simplicity.
|
|
|
|
A solution for indirect sp could be to pin the gfn, for example via
|
|
kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg. After the pinning:
|
|
|
|
- We have held the refcount of pfn; that means the pfn can not be freed and
|
|
be reused for another gfn.
|
|
- The pfn is writable and therefore it cannot be shared between different gfns
|
|
by KSM.
|
|
|
|
Then, we can ensure the dirty bitmaps is correctly set for a gfn.
|
|
|
|
2) Dirty bit tracking
|
|
|
|
In the origin code, the spte can be fast updated (non-atomically) if the
|
|
spte is read-only and the Accessed bit has already been set since the
|
|
Accessed bit and Dirty bit can not be lost.
|
|
|
|
But it is not true after fast page fault since the spte can be marked
|
|
writable between reading spte and updating spte. Like below case:
|
|
|
|
+------------------------------------------------------------------------+
|
|
| At the beginning:: |
|
|
| |
|
|
| spte.W = 0 |
|
|
| spte.Accessed = 1 |
|
|
+------------------------------------+-----------------------------------+
|
|
| CPU 0: | CPU 1: |
|
|
+------------------------------------+-----------------------------------+
|
|
| In mmu_spte_clear_track_bits():: | |
|
|
| | |
|
|
| old_spte = *spte; | |
|
|
| | |
|
|
| | |
|
|
| /* 'if' condition is satisfied. */| |
|
|
| if (old_spte.Accessed == 1 && | |
|
|
| old_spte.W == 0) | |
|
|
| spte = 0ull; | |
|
|
+------------------------------------+-----------------------------------+
|
|
| | on fast page fault path:: |
|
|
| | |
|
|
| | spte.W = 1 |
|
|
| | |
|
|
| | memory write on the spte:: |
|
|
| | |
|
|
| | spte.Dirty = 1 |
|
|
+------------------------------------+-----------------------------------+
|
|
| :: | |
|
|
| | |
|
|
| else | |
|
|
| old_spte = xchg(spte, 0ull) | |
|
|
| if (old_spte.Accessed == 1) | |
|
|
| kvm_set_pfn_accessed(spte.pfn);| |
|
|
| if (old_spte.Dirty == 1) | |
|
|
| kvm_set_pfn_dirty(spte.pfn); | |
|
|
| OOPS!!! | |
|
|
+------------------------------------+-----------------------------------+
|
|
|
|
The Dirty bit is lost in this case.
|
|
|
|
In order to avoid this kind of issue, we always treat the spte as "volatile"
|
|
if it can be updated out of mmu-lock [see spte_has_volatile_bits()]; it means
|
|
the spte is always atomically updated in this case.
|
|
|
|
3) flush tlbs due to spte updated
|
|
|
|
If the spte is updated from writable to read-only, we should flush all TLBs,
|
|
otherwise rmap_write_protect will find a read-only spte, even though the
|
|
writable spte might be cached on a CPU's TLB.
|
|
|
|
As mentioned before, the spte can be updated to writable out of mmu-lock on
|
|
fast page fault path. In order to easily audit the path, we see if TLBs needing
|
|
to be flushed caused this reason in mmu_spte_update() since this is a common
|
|
function to update spte (present -> present).
|
|
|
|
Since the spte is "volatile" if it can be updated out of mmu-lock, we always
|
|
atomically update the spte and the race caused by fast page fault can be avoided.
|
|
See the comments in spte_has_volatile_bits() and mmu_spte_update().
|
|
|
|
Lockless Access Tracking:
|
|
|
|
This is used for Intel CPUs that are using EPT but do not support the EPT A/D
|
|
bits. In this case, PTEs are tagged as A/D disabled (using ignored bits), and
|
|
when the KVM MMU notifier is called to track accesses to a page (via
|
|
kvm_mmu_notifier_clear_flush_young), it marks the PTE not-present in hardware
|
|
by clearing the RWX bits in the PTE and storing the original R & X bits in more
|
|
unused/ignored bits. When the VM tries to access the page later on, a fault is
|
|
generated and the fast page fault mechanism described above is used to
|
|
atomically restore the PTE to a Present state. The W bit is not saved when the
|
|
PTE is marked for access tracking and during restoration to the Present state,
|
|
the W bit is set depending on whether or not it was a write access. If it
|
|
wasn't, then the W bit will remain clear until a write access happens, at which
|
|
time it will be set using the Dirty tracking mechanism described above.
|
|
|
|
3. Reference
|
|
------------
|
|
|
|
``kvm_lock``
|
|
^^^^^^^^^^^^
|
|
|
|
:Type: mutex
|
|
:Arch: any
|
|
:Protects: - vm_list
|
|
- kvm_usage_count
|
|
- hardware virtualization enable/disable
|
|
:Comment: KVM also disables CPU hotplug via cpus_read_lock() during
|
|
enable/disable.
|
|
|
|
``kvm->mn_invalidate_lock``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
:Type: spinlock_t
|
|
:Arch: any
|
|
:Protects: mn_active_invalidate_count, mn_memslots_update_rcuwait
|
|
|
|
``kvm_arch::tsc_write_lock``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
:Type: raw_spinlock_t
|
|
:Arch: x86
|
|
:Protects: - kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
|
|
- tsc offset in vmcb
|
|
:Comment: 'raw' because updating the tsc offsets must not be preempted.
|
|
|
|
``kvm->mmu_lock``
|
|
^^^^^^^^^^^^^^^^^
|
|
:Type: spinlock_t or rwlock_t
|
|
:Arch: any
|
|
:Protects: -shadow page/shadow tlb entry
|
|
:Comment: it is a spinlock since it is used in mmu notifier.
|
|
|
|
``kvm->srcu``
|
|
^^^^^^^^^^^^^
|
|
:Type: srcu lock
|
|
:Arch: any
|
|
:Protects: - kvm->memslots
|
|
- kvm->buses
|
|
:Comment: The srcu read lock must be held while accessing memslots (e.g.
|
|
when using gfn_to_* functions) and while accessing in-kernel
|
|
MMIO/PIO address->device structure mapping (kvm->buses).
|
|
The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
|
|
if it is needed by multiple functions.
|
|
|
|
``kvm->slots_arch_lock``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
:Type: mutex
|
|
:Arch: any (only needed on x86 though)
|
|
:Protects: any arch-specific fields of memslots that have to be modified
|
|
in a ``kvm->srcu`` read-side critical section.
|
|
:Comment: must be held before reading the pointer to the current memslots,
|
|
until after all changes to the memslots are complete
|
|
|
|
``wakeup_vcpus_on_cpu_lock``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
:Type: spinlock_t
|
|
:Arch: x86
|
|
:Protects: wakeup_vcpus_on_cpu
|
|
:Comment: This is a per-CPU lock and it is used for VT-d posted-interrupts.
|
|
When VT-d posted-interrupts are supported and the VM has assigned
|
|
devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu
|
|
protected by blocked_vcpu_on_cpu_lock. When VT-d hardware issues
|
|
wakeup notification event since external interrupts from the
|
|
assigned devices happens, we will find the vCPU on the list to
|
|
wakeup.
|
|
|
|
``vendor_module_lock``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
:Type: mutex
|
|
:Arch: x86
|
|
:Protects: loading a vendor module (kvm_amd or kvm_intel)
|
|
:Comment: Exists because using kvm_lock leads to deadlock. cpu_hotplug_lock is
|
|
taken outside of kvm_lock, e.g. in KVM's CPU online/offline callbacks, and
|
|
many operations need to take cpu_hotplug_lock when loading a vendor module,
|
|
e.g. updating static calls.
|