linux-kernel - Re: [RFC PATCH 1/6] KVM: X86: Check root_level only in fast_pgd

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Message-ID: <YdSs7ocQoy6txrMu@google.com>
Date:   Tue, 4 Jan 2022 20:24:14 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Lai Jiangshan <laijs@...ux.alibaba.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: [RFC PATCH 1/6] KVM: X86: Check root_level only in
 fast_pgd_switch()

On Fri, Dec 10, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> If root_level >= 4, shadow_root_level must be >= 4 too.

Please explain why so that it's explicitly clear why this ok, e.g.

  Drop the shadow_root_level check when determining if a "fast" PGD switch
  is allowed, as KVM never shadows 64-bit guest pages tables with PAE paging
  (32-bit KVM doesn't support 64-bit guests).

with that:

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

> 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 11b06d536cc9..846a2e426e0b 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;
> -- 
> 2.19.1.6.gb485710b
>