diff options
Diffstat (limited to 'Documentation/virt/kvm/locking.rst')
| -rw-r--r-- | Documentation/virt/kvm/locking.rst | 76 |
1 files changed, 39 insertions, 37 deletions
diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst index 1bedd56e2fe3..693090bfc66d 100644 --- a/Documentation/virt/kvm/locking.rst +++ b/Documentation/virt/kvm/locking.rst @@ -147,49 +147,51 @@ 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. |