Re: [PATCH 2/2] kvm: nVMX: Single-step traps trump expired VMX-preemption timer

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Apr 13, 2020 at 05:09:46PM -0700, Jim Mattson wrote:
> Previously, if the hrtimer for the nested VMX-preemption timer fired
> while L0 was emulating an L2 instruction with RFLAGS.TF set, the
> synthesized single-step trap would be unceremoniously dropped when
> synthesizing the "VMX-preemption timer expired" VM-exit from L2 to L1.
> 
> To fix this, don't synthesize a "VMX-preemption timer expired" VM-exit
> from L2 to L1 when there is a pending debug trap, such as a
> single-step trap.
> 
> Fixes: f4124500c2c13 ("KVM: nVMX: Fully emulate preemption timer")
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Reviewed-by: Oliver Upton <oupton@google.com>
> Reviewed-by: Peter Shier <pshier@google.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index cbc9ea2de28f..6ab974debd44 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3690,7 +3690,9 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>  	    vmx->nested.preemption_timer_expired) {
>  		if (block_nested_events)
>  			return -EBUSY;
> -		nested_vmx_vmexit(vcpu, EXIT_REASON_PREEMPTION_TIMER, 0, 0);
> +		if (!vmx_pending_dbg_trap(vcpu))

IMO this one warrants a comment.  It's not immediately obvious that this
only applies to #DBs that are being injected into L2, and that returning
-EBUSY will do the wrong thing.

> +			nested_vmx_vmexit(vcpu, EXIT_REASON_PREEMPTION_TIMER,
> +					  0, 0);

I'd just let the "0, 0);" poke out past 80 chars.

>  		return 0;
>  	}
>  
> -- 
> 2.26.0.110.g2183baf09c-goog
>