Re: [PATCH v3 2/5] KVM: add KVM_REQ_EXIT request for userspace exit

Wanpeng Li
2015-08-20T03:55:44Z

On 8/14/15 6:08 PM, Radim Krčmář wrote:
> When userspace wants KVM to exit to userspace, it sends a signal.
> This has a disadvantage of requiring a change to the signal mask because
> the signal needs to be blocked in userspace to stay pending when sending
> to self.
>
> Using a request flag allows us to shave 200-300 cycles from every
> userspace exit and the speedup grows with NUMA because unblocking
> touches shared spinlock.
>
> The disadvantage is that it adds an overhead of one bit check for all
> kernel exits. A quick tracing shows that the ratio of userspace exits
> after boot is about 1/5 and in subsequent run of nmap and kernel compile
> has about 1/60, so the check should not regress global performance.
>
> All signal_pending() calls are userspace exit requests, so we add a
> check for KVM_REQ_EXIT there. There is one omitted call in kvm_vcpu_run
> because KVM_REQ_EXIT is implied in earlier check for requests.

Actually I see more SIGUSR1 signals are intercepted by signal_pending()
in vcpu_enter_guest() and vcpu_run() w/ win7 guest and kernel_irqchip=off.

Regards,
Wanpeng Li

>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
> arch/x86/kvm/vmx.c | 2 +-
> arch/x86/kvm/x86.c | 6 ++++++
> include/linux/kvm_host.h | 8 +++++++-
> include/uapi/linux/kvm.h | 1 +
> virt/kvm/kvm_main.c | 2 +-
> 5 files changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 40c6180a0ecb..2b789a869ef5 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5833,7 +5833,7 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
> goto out;
> }
>
> - if (signal_pending(current))
> + if (kvm_need_exit(vcpu))
> goto out;
> if (need_resched())
> schedule();
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e5850076bf7b..c3df7733af09 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6548,6 +6548,11 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
> ++vcpu->stat.signal_exits;
> break;
> }
> + if (unlikely(kvm_has_request(KVM_REQ_EXIT, vcpu))) {
> + r = 0;
> + vcpu->run->exit_reason = KVM_EXIT_REQUEST;
> + break;
> + }
> if (need_resched()) {
> srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
> cond_resched();
> @@ -6684,6 +6689,7 @@ out:
> post_kvm_run_save(vcpu);
> if (vcpu->sigset_active)
> sigprocmask(SIG_SETMASK, &sigsaved, NULL);
> + clear_bit(KVM_REQ_EXIT, &vcpu->requests);
>
> return r;
> }
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 52e388367a26..dcc57171e3ec 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -121,7 +121,7 @@ static inline bool is_error_page(struct page *page)
> #define KVM_REQ_UNHALT 6
> #define KVM_REQ_MMU_SYNC 7
> #define KVM_REQ_CLOCK_UPDATE 8
> -#define KVM_REQ_KICK 9
> +#define KVM_REQ_EXIT 9
> #define KVM_REQ_DEACTIVATE_FPU 10
> #define KVM_REQ_EVENT 11
> #define KVM_REQ_APF_HALT 12
> @@ -1104,6 +1104,12 @@ static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
> }
> }
>
> +static inline bool kvm_need_exit(struct kvm_vcpu *vcpu)
> +{
> + return signal_pending(current) ||
> + kvm_has_request(KVM_REQ_EXIT, vcpu);
> +}
> +
> extern bool kvm_rebooting;
>
> struct kvm_device {
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 26daafbba9ec..d996a7cdb4d2 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -184,6 +184,7 @@ struct kvm_s390_skeys {
> #define KVM_EXIT_SYSTEM_EVENT 24
> #define KVM_EXIT_S390_STSI 25
> #define KVM_EXIT_IOAPIC_EOI 26
> +#define KVM_EXIT_REQUEST 27
>
> /* For KVM_EXIT_INTERNAL_ERROR */
> /* Emulate instruction failed. */
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d8db2f8fce9c..347899966178 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1914,7 +1914,7 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
> }
> if (kvm_cpu_has_pending_timer(vcpu))
> return -EINTR;
> - if (signal_pending(current))
> + if (kvm_need_exit(vcpu))
> return -EINTR;
>
> return 0;