Re: [PATCH 09/10 RFC] KVM: x86: MMU: Move parent_pte handling from kvm_mmu_get_page() to link_shadow_page()

From: Paolo Bonzini
Date: Thu Nov 12 2015 - 09:27:35 EST

On 12/11/2015 12:56, Takuya Yoshikawa wrote:
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 9d21b44..f414ca6 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -598,7 +598,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
> goto out_gpte_changed;
> if (sp)
> - link_shadow_page(it.sptep, sp, PT_GUEST_ACCESSED_MASK);
> + link_shadow_page(vcpu, it.sptep, sp, PT_GUEST_ACCESSED_MASK);
> }

Here I think you can remove completely the

if (sp)
kvm_mmu_put_page(sp, it.sptep);

later in FNAME(fetch). Apart from this nit, it's okay.

On to kvm_mmu_get_page...

if (!direct) {
if (rmap_write_protect(vcpu, gfn))
if (level > PT_PAGE_TABLE_LEVEL && need_sync)
kvm_sync_pages(vcpu, gfn);

This seems fishy.

need_sync is set if sp->unsync, but then the parents have not been
unsynced yet.

On the other hand, all calls to kvm_mmu_get_page except for the
roots are followed by link_shadow_page... Perhaps if parent_pte != NULL
you can call link_shadow_page directly from kvm_mmu_get_page. The call
would go before the "if (!direct)" and it would subsume all the existing

We could probably also warn if

(parent_pte == NULL)
!= (level == vcpu->arch.mmu.root_level)

in kvm_mmu_get_page.

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