| Message ID | 20190710160734.4559-1-sean.j.christopherson@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86: Unconditionally enable irqs in guest context | expand |
On 10/07/19 18:07, Sean Christopherson wrote: > On VMX, KVM currently does not re-enable irqs until after it has exited > the guest context. As a result, a tick that fires in the window between > VM-Exit and guest_exit_irqoff() will be accounted as system time. While > said window is relatively small, it's large enough to be problematic in > some configurations, e.g. if VM-Exits are consistently occurring a hair > earlier than the tick irq. > > Intentionally toggle irqs back off so that guest_exit_irqoff() can be > used in lieu of guest_exit() in order to avoid the save/restore of flags > in guest_exit(). On my Haswell system, "nop; cli; sti" is ~6 cycles, > versus ~28 cycles for "pushf; pop <reg>; cli; push <reg>; popf". > > Fixes: f2485b3e0c6c0 ("KVM: x86: use guest_exit_irqoff") > Reported-by: Wei Yang <w90p710@gmail.com> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> > --- > arch/x86/kvm/svm.c | 10 +--------- > arch/x86/kvm/x86.c | 11 +++++++++++ > 2 files changed, 12 insertions(+), 9 deletions(-) > > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c > index 5270711e787f..98b848fcf3e3 100644 > --- a/arch/x86/kvm/svm.c > +++ b/arch/x86/kvm/svm.c > @@ -6184,15 +6184,7 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu, > > static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu) > { > - kvm_before_interrupt(vcpu); > - local_irq_enable(); > - /* > - * We must have an instruction with interrupts enabled, so > - * the timer interrupt isn't delayed by the interrupt shadow. > - */ > - asm("nop"); > - local_irq_disable(); > - kvm_after_interrupt(vcpu); > + > } > > static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu) > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 2e302e977dac..32561032f7e6 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -8042,7 +8042,18 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > > kvm_x86_ops->handle_exit_irqoff(vcpu); > > + /* > + * Consume any pending interrupts, including the possible source of > + * VM-Exit on SVM and any ticks that occur between VM-Exit and now. > + * An instruction is required after local_irq_enable() to fully unblock > + * interrupts on processors that implement an interrupt shadow, the > + * stat.exits increment will do nicely. > + */ > + kvm_before_interrupt(vcpu); > + local_irq_enable(); > ++vcpu->stat.exits; > + local_irq_disable(); > + kvm_after_interrupt(vcpu); > > guest_exit_irqoff(); > if (lapic_in_kernel(vcpu)) { > Queued, thanks. Paolo
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c index 5270711e787f..98b848fcf3e3 100644 --- a/arch/x86/kvm/svm.c +++ b/arch/x86/kvm/svm.c @@ -6184,15 +6184,7 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu, static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu) { - kvm_before_interrupt(vcpu); - local_irq_enable(); - /* - * We must have an instruction with interrupts enabled, so - * the timer interrupt isn't delayed by the interrupt shadow. - */ - asm("nop"); - local_irq_disable(); - kvm_after_interrupt(vcpu); + } static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 2e302e977dac..32561032f7e6 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8042,7 +8042,18 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) kvm_x86_ops->handle_exit_irqoff(vcpu); + /* + * Consume any pending interrupts, including the possible source of + * VM-Exit on SVM and any ticks that occur between VM-Exit and now. + * An instruction is required after local_irq_enable() to fully unblock + * interrupts on processors that implement an interrupt shadow, the + * stat.exits increment will do nicely. + */ + kvm_before_interrupt(vcpu); + local_irq_enable(); ++vcpu->stat.exits; + local_irq_disable(); + kvm_after_interrupt(vcpu); guest_exit_irqoff(); if (lapic_in_kernel(vcpu)) {
On VMX, KVM currently does not re-enable irqs until after it has exited the guest context. As a result, a tick that fires in the window between VM-Exit and guest_exit_irqoff() will be accounted as system time. While said window is relatively small, it's large enough to be problematic in some configurations, e.g. if VM-Exits are consistently occurring a hair earlier than the tick irq. Intentionally toggle irqs back off so that guest_exit_irqoff() can be used in lieu of guest_exit() in order to avoid the save/restore of flags in guest_exit(). On my Haswell system, "nop; cli; sti" is ~6 cycles, versus ~28 cycles for "pushf; pop <reg>; cli; push <reg>; popf". Fixes: f2485b3e0c6c0 ("KVM: x86: use guest_exit_irqoff") Reported-by: Wei Yang <w90p710@gmail.com> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> --- arch/x86/kvm/svm.c | 10 +--------- arch/x86/kvm/x86.c | 11 +++++++++++ 2 files changed, 12 insertions(+), 9 deletions(-)