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KVM: MMU: Explicitly set D-bit for writable spte.

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This patch avoids unnecessary dirty GPA logging to PML buffer in EPT violation
path by setting D-bit manually prior to the occurrence of the write from guest.

We only set D-bit manually in set_spte, and leave fast_page_fault path
unchanged, as fast_page_fault is very unlikely to happen in case of PML.

For the hva <-> pa change case, the spte is updated to either read-only (host
pte is read-only) or be dropped (host pte is writeable), and both cases will be
handled by above changes, therefore no change is necessary.

Signed-off-by: Kai Huang <kai.huang@linux.intel.com>
Reviewed-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Kai Huang 2015-01-28 10:54:25 +08:00 committed by Paolo Bonzini
parent f4b4b18086
commit 9b51a63024
1 changed files with 15 additions and 1 deletions
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16
arch/x86/kvm/mmu.c
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@ -2597,8 +2597,10 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
}
}
if (pte_access & ACC_WRITE_MASK)
if (pte_access & ACC_WRITE_MASK) {
mark_page_dirty(vcpu->kvm, gfn);
spte |= shadow_dirty_mask;
}
set_pte:
if (mmu_spte_update(sptep, spte))
@ -2914,6 +2916,18 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
*/
gfn = kvm_mmu_page_get_gfn(sp, sptep - sp->spt);
/*
* Theoretically we could also set dirty bit (and flush TLB) here in
* order to eliminate unnecessary PML logging. See comments in
* set_spte. But fast_page_fault is very unlikely to happen with PML
* enabled, so we do not do this. This might result in the same GPA
* to be logged in PML buffer again when the write really happens, and
* eventually to be called by mark_page_dirty twice. But it's also no
* harm. This also avoids the TLB flush needed after setting dirty bit
* so non-PML cases won't be impacted.
*
* Compare with set_spte where instead shadow_dirty_mask is set.
*/
if (cmpxchg64(sptep, spte, spte | PT_WRITABLE_MASK) == spte)
mark_page_dirty(vcpu->kvm, gfn);