linux-kernel - Re: [PATCH 1/7] KVM: VMX: read idt_vectoring

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Message-ID: <YC1Cljte2ozGSKV/@google.com>
Date:   Wed, 17 Feb 2021 08:21:42 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 1/7] KVM: VMX: read idt_vectoring_info a bit earlier

On Wed, Feb 17, 2021, Maxim Levitsky wrote:
> On Wed, 2021-02-17 at 17:06 +0100, Paolo Bonzini wrote:
> > On 17/02/21 15:57, Maxim Levitsky wrote:
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index b3e36dc3f164..e428d69e21c0 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -6921,13 +6921,15 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> > >  	if (unlikely((u16)vmx->exit_reason.basic == EXIT_REASON_MCE_DURING_VMENTRY))
> > >  		kvm_machine_check();
> > >  
> > > +	if (likely(!vmx->exit_reason.failed_vmentry))
> > > +		vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
> > > +
> > 
> > Any reason for the if?
> 
> Sean Christopherson asked me to do this to avoid updating idt_vectoring_info on failed
> VM entry, to keep things as they were logically before this patch.

Ya, specifically because the field isn't valid if VM-Enter fails.  I'm also ok
with an unconditional VMREAD if we add a comment stating that it's unnecessary
if VM-Enter failed, but faster in the common case.