linux-kernel - Re: [PATCH v7 036/102] KVM: x86/mmu: Allow non-zero value for non-present SPTE

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Message-ID: <Ysw3778RNiEwuezX@google.com>
Date:   Mon, 11 Jul 2022 14:47:11 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yuan Yao <yuan.yao@...ux.intel.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v7 036/102] KVM: x86/mmu: Allow non-zero value for
 non-present SPTE

On Mon, Jul 11, 2022, Yuan Yao wrote:
> On Fri, Jul 08, 2022 at 03:30:23PM +0000, Sean Christopherson wrote:
> > Please trim replies.
> > > I'm not sure why skip this for TDX, arch.mmu_shadow_page_cache is
> > > still used for allocating sp->spt which used to track the S-EPT in kvm
> > > for tdx guest.  Anything I missed for this ?
> >
> > Shared EPTEs need to be initialized with SUPPRESS_VE=1, otherwise not-present
> > EPT violations would be reflected into the guest by hardware as #VE exceptions.
> > This is handled by initializing page allocations via kvm_init_shadow_page() during
> > cache topup if shadow_nonpresent_value is non-zero.  In that case, telling the
> > page allocation to zero-initialize the page would be wasted effort.
> >
> > The initialization is harmless for S-EPT entries because KVM's copy of the S-EPT
> > isn't consumed by hardware, and because under the hood S-EPT entries should never
> > #VE (I forget if this is enforced by hardware or if the TDX module sets SUPPRESS_VE).
> 
> Ah I see, you're right, thanks for the explanation! I think with
> changes you suggested above the __GFP_ZERO can be removed from
> mmu_shadow_page_cache for VMs which is_tdp_mmu_enabled() is true:

Yep.

> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 8de26cbde295..0b412f3eb0c5 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6483,8 +6483,8 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
>  	vcpu->arch.mmu_page_header_cache.kmem_cache = mmu_page_header_cache;
>  	vcpu->arch.mmu_page_header_cache.gfp_zero = __GFP_ZERO;
> 
> -	if (!(tdp_enabled && shadow_nonpresent_value))
> -		vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
> +	if (!(is_tdp_mmu_enabled(vcpu->kvm))
> +	    vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
> 
>  	vcpu->arch.mmu = &vcpu->arch.root_mmu;
>  	vcpu->arch.walk_mmu = &vcpu->arch.root_mmu;