Re: [PATCH v2 4/5] KVM: add KVM_USER_EXIT vcpu ioctl for userspace exit

[Avi Kivity]

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 KrÄmÃÅ 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 KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > > ---
> > >    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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