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

[Sean Christopherson <seanjc@google.com>]

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.

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
>