[xenial][PATCH 01/15] KVM: race-free exit from KVM_RUN without POSIX signals
Jose Ricardo Ziviani
joserz at linux.vnet.ibm.com
Thu Jul 20 15:49:16 UTC 2017
From: Paolo Bonzini <pbonzini at redhat.com>
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
Reviewed-by: Radim Krčmář <rkrcmar at redhat.com>
Reviewed-by: David Hildenbrand <david at redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Jose Ricardo Ziviani <joserz at linux.vnet.ibm.com>
---
Documentation/virtual/kvm/api.txt | 13 ++++++++++++-
arch/arm/kvm/arm.c | 4 ++++
arch/mips/kvm/mips.c | 8 +++++++-
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, 40 insertions(+), 5 deletions(-)
diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 4470671..c598f88 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -3282,7 +3282,18 @@ 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 9d74464..f396cb9 100644
--- a/arch/arm/kvm/arm.c
+++ b/arch/arm/kvm/arm.c
@@ -207,6 +207,7 @@ 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 29ec9ab..5951d24 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -452,7 +452,7 @@ static void kvm_mips_check_asids(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)
@@ -464,6 +464,9 @@ 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();
@@ -489,6 +492,7 @@ 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);
@@ -1282,6 +1286,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
switch (ext) {
case KVM_CAP_ONE_REG:
case KVM_CAP_ENABLE_CAP:
+ case KVM_CAP_READONLY_MEM:
+ case KVM_CAP_SYNC_MMU:
r = 1;
break;
case KVM_CAP_COALESCED_MMIO:
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index 40a5b2d..04e6ebd 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:
@@ -1114,7 +1115,10 @@ 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 ac9eb59..6409e95 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:
@@ -2763,6 +2764,9 @@ 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 0a1904b..91af91f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2633,6 +2633,7 @@ 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:
@@ -7104,7 +7105,10 @@ 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 e003580..214eb6c 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;
@@ -873,6 +874,7 @@ struct kvm_ppc_smmu_info {
#define KVM_CAP_PPC_HTM 132
#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
--
2.7.4
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