Re: [PATCH] kvm: rename HINTS_DEDICATED to KVM_HINTS_REALTIME

Michael S. Tsirkin
2018-05-18T17:01:49Z

On Fri, May 18, 2018 at 01:04:31PM -0300, Eduardo Habkost wrote:
> CCing qemu-devel, as I'm now discussing userspace.
>
> On Thu, May 17, 2018 at 10:55:33PM +0300, Michael S. Tsirkin wrote:
> > On Thu, May 17, 2018 at 03:46:58PM -0300, Eduardo Habkost wrote:
> > > On Thu, May 17, 2018 at 05:54:24PM +0300, Michael S. Tsirkin wrote:
> > > > HINTS_DEDICATED seems to be somewhat confusing:
> > > >
> > > > Guest doesn't really care whether it's the only task running on a host
> > > > CPU as long as it's not preempted.
> > > >
> > > > And there are more reasons for Guest to be preempted than host CPU
> > > > sharing, for example, with memory overcommit it can get preempted on a
> > > > memory access, post copy migration can cause preemption, etc.
> > > >
> > > > Let's call it KVM_HINTS_REALTIME which seems to better
> > > > match what guests expect.
> > > >
> > > > Also, the flag most be set on all vCPUs - current guests assume th.
> > > > Note so in the documentation.
> > > >
> > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > ---
> > > > Documentation/virtual/kvm/cpuid.txt | 6 +++---
> > > > arch/x86/include/uapi/asm/kvm_para.h | 2 +-
> > > > arch/x86/kernel/kvm.c | 8 ++++----
> > > > 3 files changed, 8 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
> > > > index d4f33eb8..ab022dc 100644
> > > > --- a/Documentation/virtual/kvm/cpuid.txt
> > > > +++ b/Documentation/virtual/kvm/cpuid.txt
> > > > @@ -72,8 +72,8 @@ KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side
> > > >
> > > > flag || value || meaning
> > > > ==================================================================================
> > > > -KVM_HINTS_DEDICATED || 0 || guest checks this feature bit to
> > > > - || || determine if there is vCPU pinning
> > > > - || || and there is no vCPU over-commitment,
> > > > +KVM_HINTS_REALTIME || 0 || guest checks this feature bit to
> > > > + || || determine that vCPUs are never
> > > > + || || preempted for an unlimited time,
> > > > || || allowing optimizations
> > >
> > > My understanding of the original patch is that the intention is
> > > to tell the guest that it is very unlikely to be preempted,
> > > so it
> > > can choose a more appropriate spinlock implementation. This
> > > description implies that the guest will never be preempted, which
> > > is much stronger guarantee.
> >
> > Note:
> >
> > ... for an unlimited time.
>
> Which still sounds like a stronger guarantee than the original
> description. But:
>
> >
> > >
> > > Isn't this new description incompatible with existing usage of
> > > the hint, which might include people who just use vCPU pinning
> > > but no mlock?
> >
> > Without mlock you should always use pv spinlocks.
> >
> > Otherwise you risk blocking on a lock taken by
> > a VCPU that is in turn blocked on IO, where the IO
> > is not completing because CPU is being used up
> > spinning.
>
> So the stronger guarantee seems necessary.
>
> Now what should host userspace do if the user is trying to run an
> existing configuration where the CPUID hint was set but memory is
> not pinned?
>
> --
> Eduardo

As much as we'd like to be helpful and validate input, you need a real
time host too. I'm not sure how we'd find out - I suggest we do not
bother for now.

--
MST