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KVM Lock Overview
1. Acquisition Orders
(to be written)
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 only if the
shadow page table is present and it is caused by write-protect, that means
we just need change the W bit of the spte.
What we use to avoid all the race is the SPTE_HOST_WRITEABLE bit and
SPTE_MMU_WRITEABLE bit on the spte:
- SPTE_HOST_WRITEABLE means the gfn is writable on host.
- SPTE_MMU_WRITEABLE means the gfn is writable on mmu. The bit is set when
the gfn is writable on guest 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.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, 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:
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)
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, before we do cmpxchg, we call gfn_to_pfn_atomic()
to pin gfn to pfn, because after gfn_to_pfn_atomic():
- 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 that means it can not be shared between different gfns
by KSM.
Then, we can ensure the dirty bitmaps is correctly set for a gfn.
Currently, to simplify the whole things, we disable fast page fault for
indirect shadow page.
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
In mmu_spte_clear_track_bits():
old_spte = *spte;
/* 'if' condition is satisfied. */
if (old_spte.Accssed == 1 &&
old_spte.W == 0)
spte = 0ull;
on fast page fault path:
spte.W = 1
memory write on the spte:
spte.Dirty = 1
old_spte = xchg(spte, 0ull)
if (old_spte.Accssed == 1)
if (old_spte.Dirty == 1)
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 atomicly updated in this case.
3): flush tlbs due to spte updated
If the spte is updated from writable to readonly, 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 need
be flushed caused by 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
atomicly update the spte, the race caused by fast page fault can be avoided,
See the comments in spte_has_volatile_bits() and mmu_spte_update().
3. Reference
Name: kvm_lock
Type: raw_spinlock
Arch: any
Protects: - vm_list
- hardware virtualization enable/disable
Comment: 'raw' because hardware enabling/disabling must be atomic /wrt
Name: kvm_arch::tsc_write_lock
Type: raw_spinlock
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.
Name: kvm->mmu_lock
Type: spinlock_t
Arch: any
Protects: -shadow page/shadow tlb entry
Comment: it is a spinlock since it is used in mmu notifier.