Re: [PATCH 05/15] KVM: MMU: optimize to handle dirty bit

Xiao Guangrong
2011-06-08T03:14:30Z

On 06/07/2011 09:01 PM, Xiao Guangrong wrote:
> If dirty bit is not set, we can make the pte access read-only to avoid handing
> dirty bit everywhere

> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index b0c8184..67971da 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -106,6 +106,9 @@ static unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, pt_element_t gpte)
> unsigned access;
>
> access = (gpte & (PT_WRITABLE_MASK | PT_USER_MASK)) | ACC_EXEC_MASK;
> + if (!is_dirty_gpte(gpte))
> + access &= ~ACC_WRITE_MASK;
> +

Sorry, it can break something: if the gpte is not on the last level and dirty bit
is set later, below patch should fix it, i'll merge it into in the next version.

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 4287dc8..6ceb5fd 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -101,12 +101,13 @@ static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
return (ret != orig_pte);
}

-static unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, pt_element_t gpte)
+static unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, pt_element_t gpte,
+ bool last)
{
unsigned access;

access = (gpte & (PT_WRITABLE_MASK | PT_USER_MASK)) | ACC_EXEC_MASK;
- if (!is_dirty_gpte(gpte))
+ if (last && !is_dirty_gpte(gpte))
access &= ~ACC_WRITE_MASK;

#if PTTYPE == 64
@@ -230,8 +231,6 @@ walk:
pte |= PT_ACCESSED_MASK;
}

- pte_access = pt_access & FNAME(gpte_access)(vcpu, pte);
-
walker->ptes[walker->level - 1] = pte;

if ((walker->level == PT_PAGE_TABLE_LEVEL) ||
@@ -266,7 +265,7 @@ walk:
break;
}

- pt_access = pte_access;
+ pt_access &= FNAME(gpte_access)(vcpu, pte, false);
--walker->level;
}

@@ -290,6 +289,7 @@ walk:
walker->ptes[walker->level - 1] = pte;
}

+ pte_access = pt_access & FNAME(gpte_access)(vcpu, pte, true);
walker->pt_access = pt_access;
walker->pte_access = pte_access;
pgprintk("%s: pte %llx pte_access %x pt_access %x\n",
@@ -369,7 +369,7 @@ static void FNAME(update_pte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
return;

pgprintk("%s: gpte %llx spte %p\n", __func__, (u64)gpte, spte);
- pte_access = sp->role.access & FNAME(gpte_access)(vcpu, gpte);
+ pte_access = sp->role.access & FNAME(gpte_access)(vcpu, gpte, true);
pfn = gfn_to_pfn_atomic(vcpu->kvm, gpte_to_gfn(gpte));
if (mmu_invalid_pfn(pfn)) {
kvm_release_pfn_clean(pfn);
@@ -444,7 +444,8 @@ static void FNAME(pte_prefetch)(struct kvm_vcpu *vcpu, struct guest_walker *gw,
if (FNAME(prefetch_invalid_gpte)(vcpu, sp, spte, gpte))
continue;

- pte_access = sp->role.access & FNAME(gpte_access)(vcpu, gpte);
+ pte_access = sp->role.access & FNAME(gpte_access)(vcpu, gpte,
+ true);
gfn = gpte_to_gfn(gpte);
pfn = pte_prefetch_gfn_to_pfn(vcpu, gfn,
pte_access & ACC_WRITE_MASK);
@@ -790,7 +791,7 @@ static bool FNAME(sync_mmio_spte)(struct kvm_vcpu *vcpu,
if (unlikely(is_mmio_spte(*sptep))) {
gfn_t gfn = gpte_to_gfn(gpte);
unsigned access = sp->role.access & FNAME(gpte_access)(vcpu,
- gpte);
+ gpte, true);

if (gfn != get_mmio_spte_gfn(*sptep)) {
__set_spte(sptep, shadow_trap_nonpresent_pte);
@@ -868,7 +869,8 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
}

nr_present++;
- pte_access = sp->role.access & FNAME(gpte_access)(vcpu, gpte);
+ pte_access = sp->role.access & FNAME(gpte_access)(vcpu, gpte,
+ true);
host_writable = sp->spt[i] & SPTE_HOST_WRITEABLE;

set_spte(vcpu, &sp->spt[i], pte_access, 0, 0,