On Mon, May 03, 2010 at 05:19:08PM +0300, Avi Kivity wrote:
> KVM_REQ_KICK poisons vcpu->requests by having a bit set during normal
> operation. This causes the fast path check for a clear vcpu->requests
> to fail all the time, triggering tons of atomic operations.
Avi,
Do you have numbers?
> Fix by replacing KVM_REQ_KICK with a vcpu->guest_mode atomic.
>
> Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 17 ++++++++++-------
> include/linux/kvm_host.h | 1 +
> 2 files changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6b2ce1d..307094a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4499,13 +4499,15 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> if (vcpu->fpu_active)
> kvm_load_guest_fpu(vcpu);
>
> - local_irq_disable();
> + atomic_set(&vcpu->guest_mode, 1);
> + smp_wmb();
IPI can trigger here?
> - clear_bit(KVM_REQ_KICK, &vcpu->requests);
> - smp_mb__after_clear_bit();
> + local_irq_disable();
>
> - if (vcpu->requests || need_resched() || signal_pending(current)) {
> - set_bit(KVM_REQ_KICK, &vcpu->requests);
> + if (!atomic_read(&vcpu->guest_mode) || vcpu->requests
> + || need_resched() || signal_pending(current)) {
> + atomic_set(&vcpu->guest_mode, 0);
> + smp_wmb();
> local_irq_enable();
> preempt_enable();
> r = 1;
> @@ -4550,7 +4552,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> if (hw_breakpoint_active())
> hw_breakpoint_restore();
>
> - set_bit(KVM_REQ_KICK, &vcpu->requests);
> + atomic_set(&vcpu->guest_mode, 0);
> + smp_wmb();
> local_irq_enable();
>
> ++vcpu->stat.exits;
> @@ -5470,7 +5473,7 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
>
> me = get_cpu();
> if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
> - if (!test_and_set_bit(KVM_REQ_KICK, &vcpu->requests))
> + if (atomic_xchg(&vcpu->guest_mode, 0))
> smp_send_reschedule(cpu);
> put_cpu();
> }
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index ce027d5..a020fa2 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -81,6 +81,7 @@ struct kvm_vcpu {
> int vcpu_id;
> struct mutex mutex;
> int cpu;
> + atomic_t guest_mode;
> struct kvm_run *run;
> unsigned long requests;
> unsigned long guest_debug;
> --
> 1.7.0.4
>
> --
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