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authorPaolo Bonzini <pbonzini@redhat.com>2017-02-08 11:50:15 +0100
committerPaolo Bonzini <pbonzini@redhat.com>2017-02-17 12:27:37 +0100
commit460df4c1fc7c00829050c08d6368dc6e6beef307 (patch)
tree72c996f591c82ce12e2bc544ae9c9cc6e2aed81e /Documentation/virtual
parentbbd6411513aa8ef3ea02abab61318daf87c1af1e (diff)
KVM: race-free exit from KVM_RUN without POSIX signals
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@redhat.com> Reviewed-by: David Hildenbrand <david@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation/virtual')
-rw-r--r--Documentation/virtual/kvm/api.txt13
1 files changed, 12 insertions, 1 deletions
diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index e4f2cdcf78eb..069450938b79 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;