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Date:   Thu, 17 May 2018 22:55:33 +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
Subject: Re: [PATCH] kvm: rename HINTS_DEDICATED to KVM_HINTS_REALTIME
Message-ID: <20180517225316-mutt-send-email-mst@kernel.org>
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 <20180517184658.GA1543@localhost.localdomain>
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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