Re: [PATCH v2 3/3] KVM: VMX: read idt_vectoring_info a bit earlier

From: Maxim Levitsky <mlevitsk@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	x86@kernel.org, Borislav Petkov <bp@alien8.de>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Joerg Roedel <joro@8bytes.org>,
	Wanpeng Li <wanpengli@tencent.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-kernel@vger.kernel.org, Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH v2 3/3] KVM: VMX: read idt_vectoring_info a bit earlier
Date: Thu, 21 Jan 2021 19:04:00 +0200	[thread overview]
Message-ID: <c6b1dec841182178c41a564fb822a615acc97762.camel@redhat.com> (raw)
In-Reply-To: <YADhzRqE6QHmTOkx@google.com>

On Thu, 2021-01-14 at 16:29 -0800, Sean Christopherson wrote:
> On Thu, Jan 14, 2021, Maxim Levitsky wrote:
> > This allows it to be printed correctly by the trace print
> 
> It'd be helpful to explicitly say which tracepoint, and explain that the value
> is read by vmx_get_exit_info().  It's far from obvious how this gets consumed.
> 
> > that follows.
> > 
> 
> Fixes: dcf068da7eb2 ("KVM: VMX: Introduce generic fastpath handler")
> 
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 2af05d3b05909..9b6e7dbf5e2bd 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6771,6 +6771,8 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >  	}
> >  
> >  	vmx->exit_reason = vmcs_read32(VM_EXIT_REASON);
> > +	vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
> 
> Hrm, it probably makes sense to either do the VMREAD conditionally, or to
> zero idt_vectoring_info in the vmx->fail path.  I don't care about the cycles
> on VM-Exit consistency checks, just that this would hide that the field is valid
> if and only if VM-Enter fully succeeded.  A third option would be to add a
> comment saying that it's unnecessary if VM-Enter failed, but faster in the
> common case to just do the VMREAD.

Allright, I will add this.

Best regards,
	Maxim Levitsky

> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 2af05d3b0590..3c172c05570a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6774,13 +6774,15 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>         if (unlikely((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY))
>                 kvm_machine_check();
> 
> +       if (likely(!(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY)))
> +               vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
> +
>         trace_kvm_exit(vmx->exit_reason, vcpu, KVM_ISA_VMX);
> 
>         if (unlikely(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
>                 return EXIT_FASTPATH_NONE;
> 
>         vmx->loaded_vmcs->launched = 1;
> -       vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
> 
>         vmx_recover_nmi_blocking(vmx);
>         vmx_complete_interrupts(vmx);
> 
> 
> > +
> >  	if (unlikely((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY))
> >  		kvm_machine_check();
> >  
> > @@ -6780,7 +6782,6 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >  		return EXIT_FASTPATH_NONE;
> >  
> >  	vmx->loaded_vmcs->launched = 1;
> > -	vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
> >  
> >  	vmx_recover_nmi_blocking(vmx);
> >  	vmx_complete_interrupts(vmx);
> > -- 
> > 2.26.2
> > 