On 08/17/2015 04:15 PM, Paolo Bonzini wrote:
On 16/08/2015 13:27, Avi Kivity wrote:
On 08/05/2015 07:33 PM, Radim Krčmář wrote:
The guest can use KVM_USER_EXIT instead of a signal-based exiting to
userspace. Availability depends on KVM_CAP_USER_EXIT.
Only x86 is implemented so far.
Signed-off-by: Radim Krčmář <rkrc...@redhat.com>
---
v2:
* use vcpu ioctl instead of vm one [4/5]
* shrink kvm_user_exit from 64 to 32 bytes [4/5]
Documentation/virtual/kvm/api.txt | 30 ++++++++++++++++++++++++++++++
arch/x86/kvm/x86.c | 24 ++++++++++++++++++++++++
include/uapi/linux/kvm.h | 7 +++++++
virt/kvm/kvm_main.c | 5 +++--
4 files changed, 64 insertions(+), 2 deletions(-)
diff --git a/Documentation/virtual/kvm/api.txt
b/Documentation/virtual/kvm/api.txt
index 3c714d43a717..c5844f0b8e7c 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -3020,6 +3020,36 @@ Returns: 0 on success, -1 on error
Queues an SMI on the thread's vcpu.
+
+4.97 KVM_USER_EXIT
+
+Capability: KVM_CAP_USER_EXIT
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_user_exit (in)
+Returns: 0 on success,
+ -EFAULT if the parameter couldn't be read,
+ -EINVAL if 'reserved' is not zeroed,
+
+struct kvm_user_exit {
+ __u8 reserved[32];
+};
+
+The ioctl is asynchronous to VCPU execution and can be issued from
all threads.
+format
This breaks an invariant of vcpu ioctls, and also forces a cacheline
bounce when we fget() the vcpu fd.
KVM_USER_EXIT in practice should be so rare (at least with in-kernel
LAPIC) that I don't think this matters. KVM_USER_EXIT is relatively
uninteresting, it only exists to provide an alternative to signals that
doesn't require expensive atomics on each and every KVM_RUN. :(
Ah, so the idea is to remove the cost of changing the signal mask?
Yes, although it looks like a thread-local operation, it takes a
process-wide lock.
I expect most user wakeups are via irqfd, so indeed the performance of
KVM_USER_EXIT is uninteresting.
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