[PATCH] KVM: race-free exit from KVM_RUN without POSIX signals

From: Paolo Bonzini
Date: Wed Feb 15 2017 - 09:43:52 EST

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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@xxxxxxxxxx>
---
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

Next message: Thomas Gleixner: "[PATCH] genirq: Reenable shared irq debugging in request_*_irq()"
Previous message: Josh Poimboeuf: "Re: [PATCH v5 01/15] stacktrace/x86: add function for detecting reliable stack traces"
Next in thread: Christian Borntraeger: "Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals"
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