linux-kernel - Re: [PATCH v3 10/11] KVM: VMX: Skip additional Hyper-V TLB EPTP flushes if one fails

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Message-ID: <87zh3myh6p.fsf@vitty.brq.redhat.com>
Date:   Thu, 12 Nov 2020 11:59:10 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...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
Subject: Re: [PATCH v3 10/11] KVM: VMX: Skip additional Hyper-V TLB EPTP
 flushes if one fails

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Skip additional EPTP flushes if one fails when processing EPTPs for
> Hyper-V's paravirt TLB flushing.  If _any_ flush fails, KVM falls back
> to a full global flush, i.e. additional flushes are unnecessary (and
> will likely fail anyways).
>
> Continue processing the loop unless a mismatch was already detected,
> e.g. to handle the case where the first flush fails and there is a
> yet-to-be-detected mismatch.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 5b7c5b2fd2c7..40a67dd45c8c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -528,7 +528,15 @@ static int hv_remote_flush_tlb_with_range(struct kvm *kvm,
>  			if (++nr_unique_valid_eptps == 1)
>  				kvm_vmx->hv_tlb_eptp = tmp_eptp;
>  
> -			ret |= hv_remote_flush_eptp(tmp_eptp, range);
> +			if (!ret)
> +				ret = hv_remote_flush_eptp(tmp_eptp, range);
> +
> +			/*
> +			 * Stop processing EPTPs if a failure occurred and
> +			 * there is already a detected EPTP mismatch.
> +			 */
> +			if (ret && nr_unique_valid_eptps > 1)
> +				break;
>  		}
>  
>  		/*

This should never happen (famous last words) but why not optimize the
impossibility :-)

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly