Re: [patch 03/10] KVM: MMU: do not write-protect large mappings

Avi Kivity
2008-09-20T00:29:24Z

Marcelo Tosatti wrote:
> There is not much point in write protecting large mappings. This
> can only happen when a page is shadowed during the window between
> is_largepage_backed and mmu_lock acquision. Zap the entry instead, so
> the next pagefault will find a shadowed page via is_largepage_backed and
> fallback to 4k translations.
>
> Simplifies out of sync shadow.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> Index: kvm/arch/x86/kvm/mmu.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/mmu.c
> +++ kvm/arch/x86/kvm/mmu.c
> @@ -1180,11 +1180,16 @@ static int set_spte(struct kvm_vcpu *vcp
> || (write_fault && !is_write_protection(vcpu) && !user_fault)) {
> struct kvm_mmu_page *shadow;
>
> + if (largepage && has_wrprotected_page(vcpu->kvm, gfn)) {
> + ret = 1;
> + spte = shadow_trap_nonpresent_pte;
> + goto set_pte;
> + }
> +
> spte |= PT_WRITABLE_MASK;
>
> shadow = kvm_mmu_lookup_page(vcpu->kvm, gfn);
> - if (shadow ||
> - (largepage && has_wrprotected_page(vcpu->kvm, gfn))) {
> + if (shadow) {
> pgprintk("%s: found shadow page for %lx, marking ro\n",
> __func__, gfn);
> ret = 1;
> @@ -1197,6 +1202,7 @@ static int set_spte(struct kvm_vcpu *vcp
> if (pte_access & ACC_WRITE_MASK)
> mark_page_dirty(vcpu->kvm, gfn);
>
> +set_pte:
> set_shadow_pte(shadow_pte, spte);
> return ret;
> }
>

Don't we need to drop a reference to the page?

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.