Re: x86: kvm: Revert "remove sched notifier for cross-cpu migrations"

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: kvm list <kvm@vger.kernel.org>, Radim Krcmar <rkrcmar@redhat.com>,
	stable <stable@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: x86: kvm: Revert "remove sched notifier for cross-cpu migrations"
Date: Wed, 25 Mar 2015 19:41:45 -0300	[thread overview]
Message-ID: <20150325224145.GA5928@amt.cnet> (raw)
In-Reply-To: <CALCETrU5jdzp2MYOMobgO8MKtDyXsMFCZsJWQFhHnv_pxQbaxQ@mail.gmail.com>

On Wed, Mar 25, 2015 at 03:33:10PM -0700, Andy Lutomirski wrote:
> On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@redhat.com> wrote:
> >
> > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Krčmář wrote:
> > > 2015-03-25 12:08+0100, Radim Krčmář:
> > > > Reverting the patch protects us from any migration, but I don't think we
> > > > need to care about changing VCPUs as long as we read a consistent data
> > > > from kvmclock.  (VCPU can change outside of this loop too, so it doesn't
> > > > matter if we return a value not fit for this VCPU.)
> > > >
> > > > I think we could drop the second __getcpu if our kvmclock was being
> > > > handled better;  maybe with a patch like the one below:
> > >
> > > The second __getcpu is not neccessary, but I forgot about rdtsc.
> > > We need to either use rtdscp, know the host has synchronized tsc, or
> > > monitor VCPU migrations.  Only the last one works everywhere.
> >
> > The vdso code is only used if host has synchronized tsc.
> >
> > But you have to handle the case where host goes from synchronized tsc to
> > unsynchronized tsc (see the clocksource notifier in the host side).
> >
> 
> Can't we change the host to freeze all vcpus and clear the stable bit
> on all of them if this happens?  This would simplify and speed up
> vclock_gettime.
> 
> --Andy

Seems interesting to do on 512-vcpus, but sure, could be done.