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Date:   Fri, 18 May 2018 20:01:49 +0300
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Eduardo Habkost <ehabkost@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-doc@vger.kernel.org,
        qemu-devel@nongnu.org
Subject: Re: [PATCH] kvm: rename HINTS_DEDICATED to KVM_HINTS_REALTIME
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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