Re: [PATCH] KVM: X86: Properly account for guest CPU time when considering context tracking

[Sean Christopherson <seanjc@google.com>]

+Thomas

On Mon, Mar 29, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> The bugzilla https://bugzilla.kernel.org/show_bug.cgi?id=209831 
> reported that the guest time remains 0 when running a while true 
> loop in the guest.
> 
> The commit 87fa7f3e98a131 ("x86/kvm: Move context tracking where it 
> belongs") moves guest_exit_irqoff() close to vmexit breaks the 
> tick-based time accouting when the ticks that happen after IRQs are 
> disabled are incorrectly accounted to the host/system time. This is 
> because we exit the guest state too early.
> 
> vtime-based time accounting is tied to context tracking, keep the 
> guest_exit_irqoff() around vmexit code when both vtime-based time 
> accounting and specific cpu is context tracking mode active. 
> Otherwise, leave guest_exit_irqoff() after handle_exit_irqoff() 
> and explicit IRQ window for tick-based time accouting.
> 
> Fixes: 87fa7f3e98a131 ("x86/kvm: Move context tracking where it belongs")
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/svm/svm.c | 3 ++-
>  arch/x86/kvm/vmx/vmx.c | 3 ++-
>  arch/x86/kvm/x86.c     | 2 ++
>  3 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 58a45bb..55fb5ce 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3812,7 +3812,8 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
>  	 * into world and some more.
>  	 */
>  	lockdep_hardirqs_off(CALLER_ADDR0);
> -	guest_exit_irqoff();
> +	if (vtime_accounting_enabled_this_cpu())
> +		guest_exit_irqoff();
>  
>  	instrumentation_begin();
>  	trace_hardirqs_off_finish();
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 32cf828..85695b3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6689,7 +6689,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
>  	 * into world and some more.
>  	 */
>  	lockdep_hardirqs_off(CALLER_ADDR0);
> -	guest_exit_irqoff();
> +	if (vtime_accounting_enabled_this_cpu())
> +		guest_exit_irqoff();

This looks ok, as CONFIG_CONTEXT_TRACKING and CONFIG_VIRT_CPU_ACCOUNTING_GEN are
selected by CONFIG_NO_HZ_FULL=y, and can't be enabled independently, e.g. the
rcu_user_exit() call won't be delayed because it will never be called in the
!vtime case.  But it still feels wrong poking into those details, e.g. it'll
be weird and/or wrong guest_exit_irqoff() gains stuff that isn't vtime specific.
Maybe that will never happen though?  And of course, my hack alternative also
pokes into the details[*].

Thomas, do you have an input on the least awful way to handle this?  My horrible
hack was to force PF_VCPU around the window where KVM handles IRQs after guest
exit.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d9f931c63293..6ddf341cd755 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9118,6 +9118,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	vcpu->mode = OUTSIDE_GUEST_MODE;
 	smp_wmb();

+	/*
+	 * Temporarily pretend this task is running a vCPU when potentially
+	 * processing an IRQ exit, including the below opening of an IRQ
+	 * window.  Tick-based accounting of guest time relies on PF_VCPU
+	 * being set when the tick IRQ handler runs.
+	 */
+	current->flags |= PF_VCPU;
 	static_call(kvm_x86_handle_exit_irqoff)(vcpu);

 	/*
@@ -9132,6 +9139,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	++vcpu->stat.exits;
 	local_irq_disable();
 	kvm_after_interrupt(vcpu);
+	current->flags &= ~PF_VCPU;

 	if (lapic_in_kernel(vcpu)) {
 		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;

[*]https://lkml.kernel.org/r/20210206004218.312023-1-seanjc@google.com

>  	instrumentation_begin();
>  	trace_hardirqs_off_finish();
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fe806e8..234c8b3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9185,6 +9185,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	++vcpu->stat.exits;
>  	local_irq_disable();
>  	kvm_after_interrupt(vcpu);
> +	if (!vtime_accounting_enabled_this_cpu())
> +		guest_exit_irqoff();
>  
>  	if (lapic_in_kernel(vcpu)) {
>  		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
> -- 
> 2.7.4
>