linux-kernel - Re: [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check

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Message-ID: <acb21d4b-679c-69ee-584a-85a6723ace96@redhat.com>
Date:   Wed, 23 Jun 2021 15:26:32 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU
 check

On 22/06/21 09:24, Sean Christopherson wrote:
> Treat a NULL shadow page in the "is a TDP MMU" check as valid, non-TDP
> root.  KVM uses a "direct" PAE paging MMU when TDP is disabled and the
> guest is running with paging disabled.  In that case, root_hpa points at
> the pae_root page (of which only 32 bytes are used), not a standard
> shadow page, and the WARN fires (a lot).
> 
> Fixes: 0b873fd7fb53 ("KVM: x86/mmu: Remove redundant is_tdp_mmu_enabled check")
> Cc: David Matlack <dmatlack@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/mmu/tdp_mmu.h | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)

Queued, thanks.

Paolo

> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index b981a044ab55..1cae4485b3bc 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -94,11 +94,13 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
>   	if (WARN_ON(!VALID_PAGE(hpa)))
>   		return false;
>   
> +	/*
> +	 * A NULL shadow page is legal when shadowing a non-paging guest with
> +	 * PAE paging, as the MMU will be direct with root_hpa pointing at the
> +	 * pae_root page, not a shadow page.
> +	 */
>   	sp = to_shadow_page(hpa);
> -	if (WARN_ON(!sp))
> -		return false;
> -
> -	return is_tdp_mmu_page(sp) && sp->root_count;
> +	return sp && is_tdp_mmu_page(sp) && sp->root_count;
>   }
>   #else
>   static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }
>