linux-kernel - Re: [PATCH v3 08/37] KVM: VMX: Skip global INVVPID fallback if vpid==0 in vpid_sync

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Message-ID: <877dz87p8q.fsf@vitty.brq.redhat.com>
Date:   Wed, 25 Mar 2020 10:33:41 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v3 08/37] KVM: VMX: Skip global INVVPID fallback if vpid==0 in vpid_sync_context()

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

> Skip the global INVVPID in the unlikely scenario that vpid==0 and the
> SINGLE_CONTEXT variant of INVVPID is unsupported.  If vpid==0, there's
> no need to INVVPID as it's impossible to do VM-Enter with VPID enabled
> and vmcs.VPID==0, i.e. there can't be any TLB entries for the vCPU with
> vpid==0.  The fact that the SINGLE_CONTEXT variant isn't supported is
> irrelevant.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/ops.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/ops.h b/arch/x86/kvm/vmx/ops.h
> index 45eaedee2ac0..33645a8e5463 100644
> --- a/arch/x86/kvm/vmx/ops.h
> +++ b/arch/x86/kvm/vmx/ops.h
> @@ -285,7 +285,7 @@ static inline void vpid_sync_context(int vpid)
>  {
>  	if (cpu_has_vmx_invvpid_single())
>  		vpid_sync_vcpu_single(vpid);
> -	else
> +	else if (vpid != 0)
>  		vpid_sync_vcpu_global();
>  }

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

(personally, I also prefer 'vpid !=0' to '!vpid', however, nested.c
uses expressions like '&& !vmcs12->virtual_processor_id' instead...)

-- 
Vitaly