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Diffstat (limited to 'Documentation/virt/kvm/locking.rst')
| -rw-r--r-- | Documentation/virt/kvm/locking.rst | 84 |
1 files changed, 43 insertions, 41 deletions
diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst index 1bedd56e2fe3..ae8bce7fecbe 100644 --- a/Documentation/virt/kvm/locking.rst +++ b/Documentation/virt/kvm/locking.rst @@ -135,8 +135,8 @@ 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 -gfn_to_pfn_memslot_atomic, before the cmpxchg. After the pinning: +A solution for indirect sp could be to pin the gfn 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. @@ -147,54 +147,56 @@ 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 +In the original 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!!! | | -+------------------------------------+-----------------------------------+ ++-------------------------------------------------------------------------+ +| At the beginning:: | +| | +| spte.W = 0 | +| spte.Accessed = 1 | ++-------------------------------------+-----------------------------------+ +| CPU 0: | CPU 1: | ++-------------------------------------+-----------------------------------+ +| In mmu_spte_update():: | | +| | | +| old_spte = *spte; | | +| | | +| | | +| /* 'if' condition is satisfied. */ | | +| if (old_spte.Accessed == 1 && | | +| old_spte.W == 0) | | +| spte = new_spte; | | ++-------------------------------------+-----------------------------------+ +| | on fast page fault path:: | +| | | +| | spte.W = 1 | +| | | +| | memory write on the spte:: | +| | | +| | spte.Dirty = 1 | ++-------------------------------------+-----------------------------------+ +| :: | | +| | | +| else | | +| old_spte = xchg(spte, new_spte);| | +| if (old_spte.Accessed && | | +| !new_spte.Accessed) | | +| flush = true; | | +| if (old_spte.Dirty && | | +| !new_spte.Dirty) | | +| flush = true; | | +| 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 +if it can be updated out of mmu-lock [see spte_needs_atomic_update()]; it means the spte is always atomically updated in this case. 3) flush tlbs due to spte updated @@ -210,7 +212,7 @@ 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(). +See the comments in spte_needs_atomic_update() and mmu_spte_update(). Lockless Access Tracking: |