Re: [kvm PATCH 2/2] KVM: nVMX: Don't clobber preemption timer in the VMCS12 before L2 ran

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

On Fri, Apr 17, 2020 at 11:34:52AM -0700, Makarand Sonare wrote:
> Don't clobber the VMX-preemption timer value in the VMCS12 during
> migration on the source while handling an L1 VMLAUNCH/VMRESUME but
> before L2 ran. In that case the VMCS12 preemption timer value
> should not be touched as it will be restarted on the target
> from its original value. This emulates migration occurring while L1
> awaits completion of its VMLAUNCH/VMRESUME instruction.
> 
> Signed-off-by: Makarand Sonare <makarandsonare@google.com>
> Signed-off-by: Peter Shier <pshier@google.com>

The SOB tags are reversed, i.e. Peter's should be first to show that he
wrote the patch and then transfered it to you for upstreaming.

> Change-Id: I376d151585d4f1449319f7512151f11bbf08c5bf
> ---
>  arch/x86/kvm/vmx/nested.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 5365d7e5921ea..66155e9114114 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3897,11 +3897,13 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
>  		vmcs12->guest_activity_state = GUEST_ACTIVITY_ACTIVE;
>  
>  	if (nested_cpu_has_preemption_timer(vmcs12)) {
> -		vmx->nested.preemption_timer_remaining =
> -			vmx_get_preemption_timer_value(vcpu);
> -		if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_VMX_PREEMPTION_TIMER)
> -			vmcs12->vmx_preemption_timer_value =
> -				vmx->nested.preemption_timer_remaining;
> +		if (!vmx->nested.nested_run_pending) {
> +			vmx->nested.preemption_timer_remaining =
> +				vmx_get_preemption_timer_value(vcpu);
> +			if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_VMX_PREEMPTION_TIMER)
> +				vmcs12->vmx_preemption_timer_value =
> +					vmx->nested.preemption_timer_remaining;
> +			}

This indentation is messed up, the closing brace is for !nested_run_pending,
but it's aligned with (vm_exit_controls & ..._PREEMPTION_TIMER).

Even better than fixing the indentation would be to include !nested_run_pending
in the top-level if statement, which reduces the nesting level and produces
a much cleaner diff, e.g.

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 409a39af121f..7dd6440425ab 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3951,7 +3951,8 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
        else
                vmcs12->guest_activity_state = GUEST_ACTIVITY_ACTIVE;

-       if (nested_cpu_has_preemption_timer(vmcs12)) {
+       if (nested_cpu_has_preemption_timer(vmcs12) &&
+           !vmx->nested.nested_run_pending) {
                vmx->nested.preemption_timer_remaining =
                        vmx_get_preemption_timer_value(vcpu);
                if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_VMX_PREEMPTION_TIMER)

>  	}
>  
>  	/*
> -- 
> 2.26.1.301.g55bc3eb7cb9-goog
>