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

From: Marcelo Tosatti <mtosatti@redhat.com>
To: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	kvm-devel <kvm@vger.kernel.org>, 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 18:28:51 -0300	[thread overview]
Message-ID: <20150325212851.GB3649@amt.cnet> (raw)
In-Reply-To: <20150325125212.GC21710@potion.brq.redhat.com>

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).