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Date: Thu, 16 Feb 2017 17:19:53 +0100
From: Radim Krčmář <rkrcmar@...hat.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
christoffer.dall@...aro.org, marc.zyngier@....com,
james.hogan@...tec.com, paulus@...ba.org, borntraeger@...ibm.com,
cornelia.huck@...ibm.com, kvmarm@...ts.cs.columbia.edu,
kvm-ppc@...r.kernel.org
Subject: Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals
2017-02-15 15:43+0100, Paolo Bonzini:
> The purpose of the KVM_SET_SIGNAL_MASK API is to let userspace "kick"
> a VCPU out of KVM_RUN through a POSIX signal. A signal is attached
> to a dummy signal handler; by blocking the signal outside KVM_RUN and
> unblocking it inside, this possible race is closed:
>
> VCPU thread service thread
> --------------------------------------------------------------
> check flag
> set flag
> raise signal
> (signal handler does nothing)
> KVM_RUN
>
> However, one issue with KVM_SET_SIGNAL_MASK is that it has to take
> tsk->sighand->siglock on every KVM_RUN. This lock is often on a
> remote NUMA node, because it is on the node of a thread's creator.
> Taking this lock can be very expensive if there are many userspace
> exits (as is the case for SMP Windows VMs without Hyper-V reference
> time counter).
>
> As an alternative, we can put the flag directly in kvm_run so that
> KVM can see it:
>
> VCPU thread service thread
> --------------------------------------------------------------
> raise signal
> signal handler
> set run->immediate_exit
> KVM_RUN
> check run->immediate_exit
>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
The old immediate exit with signal did more work, but none of it should
affect user-space, so it looks like another minor optimization,
Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>
> change from RFC:
> - implement in each architecture to ensure MMIO is completed
> [Radim]
> - do not clear the flag [David Hildenbrand, offlist]
>
> Documentation/virtual/kvm/api.txt | 13 ++++++++++++-
> arch/arm/kvm/arm.c | 4 ++++
> arch/mips/kvm/mips.c | 7 ++++++-
> arch/powerpc/kvm/powerpc.c | 6 +++++-
> arch/s390/kvm/kvm-s390.c | 4 ++++
> arch/x86/kvm/x86.c | 6 +++++-
> include/uapi/linux/kvm.h | 4 +++-
> 7 files changed, 39 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index e4f2cdcf78eb..925b1b6be073 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -3389,7 +3389,18 @@ struct kvm_run {
> Request that KVM_RUN return when it becomes possible to inject external
> interrupts into the guest. Useful in conjunction with KVM_INTERRUPT.
>
> - __u8 padding1[7];
> + __u8 immediate_exit;
> +
> +This field is polled once when KVM_RUN starts; if non-zero, KVM_RUN
> +exits immediately, returning -EINTR. In the common scenario where a
> +signal is used to "kick" a VCPU out of KVM_RUN, this field can be used
> +to avoid usage of KVM_SET_SIGNAL_MASK, which has worse scalability.
> +Rather than blocking the signal outside KVM_RUN, userspace can set up
> +a signal handler that sets run->immediate_exit to a non-zero value.
> +
> +This field is ignored if KVM_CAP_IMMEDIATE_EXIT is not available.
> +
> + __u8 padding1[6];
>
> /* out */
> __u32 exit_reason;
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 21c493a9e5c9..c9a2103faeb9 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -206,6 +206,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_ARM_PSCI_0_2:
> case KVM_CAP_READONLY_MEM:
> case KVM_CAP_MP_STATE:
> + case KVM_CAP_IMMEDIATE_EXIT:
> r = 1;
> break;
> case KVM_CAP_COALESCED_MMIO:
> @@ -604,6 +605,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> return ret;
> }
>
> + if (run->immediate_exit)
> + return -EINTR;
> +
> if (vcpu->sigset_active)
> sigprocmask(SIG_SETMASK, &vcpu->sigset, &sigsaved);
>
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index 31ee5ee0010b..ed81e5ac1426 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -397,7 +397,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>
> int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> {
> - int r = 0;
> + int r = -EINTR;
> sigset_t sigsaved;
>
> if (vcpu->sigset_active)
> @@ -409,6 +409,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> vcpu->mmio_needed = 0;
> }
>
> + if (run->immediate_exit)
> + goto out;
> +
> lose_fpu(1);
>
> local_irq_disable();
> @@ -429,6 +432,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> guest_exit_irqoff();
> local_irq_enable();
>
> +out:
> if (vcpu->sigset_active)
> sigprocmask(SIG_SETMASK, &sigsaved, NULL);
>
> @@ -1021,6 +1025,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_ENABLE_CAP:
> case KVM_CAP_READONLY_MEM:
> case KVM_CAP_SYNC_MMU:
> + case KVM_CAP_IMMEDIATE_EXIT:
> r = 1;
> break;
> case KVM_CAP_COALESCED_MMIO:
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index 2b3e4e620078..1fe1391ba2c2 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -511,6 +511,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_ONE_REG:
> case KVM_CAP_IOEVENTFD:
> case KVM_CAP_DEVICE_CTRL:
> + case KVM_CAP_IMMEDIATE_EXIT:
> r = 1;
> break;
> case KVM_CAP_PPC_PAIRED_SINGLES:
> @@ -1117,7 +1118,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> #endif
> }
>
> - r = kvmppc_vcpu_run(run, vcpu);
> + if (run->immediate_exit)
> + r = -EINTR;
> + else
> + r = kvmppc_vcpu_run(run, vcpu);
>
> if (vcpu->sigset_active)
> sigprocmask(SIG_SETMASK, &sigsaved, NULL);
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 502de74ea984..99e35fe0dea8 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -370,6 +370,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_S390_IRQCHIP:
> case KVM_CAP_VM_ATTRIBUTES:
> case KVM_CAP_MP_STATE:
> + case KVM_CAP_IMMEDIATE_EXIT:
> case KVM_CAP_S390_INJECT_IRQ:
> case KVM_CAP_S390_USER_SIGP:
> case KVM_CAP_S390_USER_STSI:
> @@ -2798,6 +2799,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> int rc;
> sigset_t sigsaved;
>
> + if (kvm_run->immediate_exit)
> + return -EINTR;
> +
> if (guestdbg_exit_pending(vcpu)) {
> kvm_s390_prepare_debug_exit(vcpu);
> return 0;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 63a89a51dcc9..2a0974383ffe 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2672,6 +2672,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_DISABLE_QUIRKS:
> case KVM_CAP_SET_BOOT_CPU_ID:
> case KVM_CAP_SPLIT_IRQCHIP:
> + case KVM_CAP_IMMEDIATE_EXIT:
> #ifdef CONFIG_KVM_DEVICE_ASSIGNMENT
> case KVM_CAP_ASSIGN_DEV_IRQ:
> case KVM_CAP_PCI_2_3:
> @@ -7202,7 +7203,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> } else
> WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
>
> - r = vcpu_run(vcpu);
> + if (kvm_run->immediate_exit)
> + r = -EINTR;
> + else
> + r = vcpu_run(vcpu);
>
> out:
> post_kvm_run_save(vcpu);
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 7964b970b9ad..f51d5082a377 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -218,7 +218,8 @@ struct kvm_hyperv_exit {
> struct kvm_run {
> /* in */
> __u8 request_interrupt_window;
> - __u8 padding1[7];
> + __u8 immediate_exit;
> + __u8 padding1[6];
>
> /* out */
> __u32 exit_reason;
> @@ -881,6 +882,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_SPAPR_RESIZE_HPT 133
> #define KVM_CAP_PPC_MMU_RADIX 134
> #define KVM_CAP_PPC_MMU_HASH_V3 135
> +#define KVM_CAP_IMMEDIATE_EXIT 136
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> --
> 1.8.3.1
>
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