linux-kernel - Re: [PATCH 11/12] KVM: X86: Check root_level only in fast_pgd

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Message-ID: <e72acd9e-eb0d-8060-c89e-b8804d0b0305@redhat.com>
Date:   Fri, 26 Nov 2021 13:56:02 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        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>
Subject: Re: [PATCH 11/12] KVM: X86: Check root_level only in
 fast_pgd_switch()

On 11/24/21 13:20, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> If root_level >= 4, shadow_root_level must be >= 4 too.
> Checking only root_level can reduce a check.
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 9fb9927264d8..1dc8bfd12ecd 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4136,8 +4136,7 @@ static bool fast_pgd_switch(struct kvm_vcpu *vcpu, gpa_t new_pgd,
>   	 * having to deal with PDPTEs. We may add support for 32-bit hosts/VMs
>   	 * later if necessary.
>   	 */
> -	if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
> -	    mmu->root_level >= PT64_ROOT_4LEVEL)
> +	if (mmu->root_level >= PT64_ROOT_4LEVEL)
>   		return cached_root_available(vcpu, new_pgd, new_role);
>   
>   	return false;
> 

Hmm, I think this is more confusing.  I *think* that adding support for 
PAE would be mostly an issue with the guest PDPTRs, and not with the 
shadow PDPTRs, but without thinking more about it I'm leaning towards 
not applying this patch.

Paolo