linux-kernel - Re: [PATCH v3 2/5] KVM: add KVM_REQ

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Date:	Thu, 20 Aug 2015 11:55:37 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	linux-kernel@...r.kernel.org
CC:	kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v3 2/5] KVM: add KVM_REQ_EXIT request for userspace exit

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@...hat.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;

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