linux-kernel - Re: [PATCH 1/7] kvm: x86/mmu: Use KVM_MMU_ROOT_XXX for kvm_mmu_invalidate

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Date:   Fri, 3 Feb 2023 20:54:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org
Subject: Re: [PATCH 1/7] kvm: x86/mmu: Use KVM_MMU_ROOT_XXX for
 kvm_mmu_invalidate_gva()

On Fri, Feb 03, 2023, Lai Jiangshan wrote:
> On Thu, Feb 2, 2023 at 9:21 AM Sean Christopherson <seanjc@...gle.com> wrote:
> 
> >
> > This is logically correct, but there's potential (weird) functional change here.
> > If this is called with an invalid root, then KVM will invalidate the GVA in all
> > roots prior to this patch, but in no roots after this patch.
> >
> > I _think_ it should be impossible get here with an invalid root.  Can you try
> > adding a prep patch to assert that the root is valid so that this patch can
> > reasonably assert that there's no functional change?
> >
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 508074e47bc0..fffd9b610196 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -792,6 +792,8 @@ void kvm_inject_emulated_page_fault(struct kvm_vcpu *vcpu,
> >         fault_mmu = fault->nested_page_fault ? vcpu->arch.mmu :
> >                                                vcpu->arch.walk_mmu;
> >
> > +       WARN_ON_ONCE(!VALID_PAGE(fault_mmu->root.hpa));
> > +
> 
> I've been updating the patches as per your suggestions.
> 
> And I suddenly realized that when fault->nested_page_fault=false
> with nested EPT, fault_mmu->root.hpa is always unset.
> 
> fault_mmu->root.hpa is just meaningless when fault_mmu is not
> vcpu->arch.mmu.

Right, because there's no KVM-managed MMU. 

> I will add it as one of the reasons for replacing the argument
> with KVM_MMU_ROOT_XXX.

And maybe call out that when using walk_mmu, the ->invlpg() implementation is
NULL, i.e. using CURRENT root is a nop.

Thanks!