linux-kernel - Re: [PATCH] KVM: x86/mmu: preserve pending TLB flush across calls to kvm_tdp_mmu_zap

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Date:   Tue, 6 Apr 2021 18:59:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: preserve pending TLB flush across calls to
 kvm_tdp_mmu_zap_sp

On Tue, Apr 06, 2021, Paolo Bonzini wrote:
> Right now, if a call to kvm_tdp_mmu_zap_sp returns false, the caller
> will skip the TLB flush, which is wrong.  There are two ways to fix
> it:
> 
> - since kvm_tdp_mmu_zap_sp will not yield and therefore will not flush
>   the TLB itself, we could change the call to kvm_tdp_mmu_zap_sp to
>   use "flush |= ..."
> 
> - or we can chain the flush argument through kvm_tdp_mmu_zap_sp down
>   to __kvm_tdp_mmu_zap_gfn_range.
> 
> This patch does the former to simplify application to stable kernels.

Eh, that and passing flush down the stack is pointless because kvm_tdp_mmu_zap_sp()
will never yield.  If you want to justify |= over passing flush, it probably
makes sense to link to the discussion that led to me changing from passing flush
to accumulating the result (well, tried to, doh).

https://lkml.kernel.org/r/20210319232006.3468382-3-seanjc@google.com

> Cc: seanjc@...gle.com
> Fixes: 048f49809c526 ("KVM: x86/mmu: Ensure TLBs are flushed for TDP MMU during NX zapping")
> Cc: <stable@...r.kernel.org> # 5.10.x: 048f49809c: KVM: x86/mmu: Ensure TLBs are flushed for TDP MMU during NX zapping
> Cc: <stable@...r.kernel.org> # 5.10.x: 33a3164161: KVM: x86/mmu: Don't allow TDP MMU to yield when recovering NX pages
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

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

> ---
>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 486aa94ecf1d..951dae4e7175 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5906,7 +5906,7 @@ static void kvm_recover_nx_lpages(struct kvm *kvm)
>  				      lpage_disallowed_link);
>  		WARN_ON_ONCE(!sp->lpage_disallowed);
>  		if (is_tdp_mmu_page(sp)) {
> -			flush = kvm_tdp_mmu_zap_sp(kvm, sp);
> +			flush |= kvm_tdp_mmu_zap_sp(kvm, sp);
>  		} else {
>  			kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
>  			WARN_ON_ONCE(sp->lpage_disallowed);
> -- 
> 2.26.2
>