Re: [PATCH] kvm: rename HINTS_DEDICATED to KVM_HINTS_REALTIME

Michael S. Tsirkin
2018-05-17T19:55:33Z

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 this.
> > 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.

>
> 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.

> --
> Eduardo