+Thomas On Mon, Mar 29, 2021, Wanpeng Li wrote: > From: Wanpeng Li <wanpengli@xxxxxxxxxxx> > > 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@xxxxxxxxxx> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx> > --- > 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@xxxxxxxxxx > 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 >
