Re: [GIT PULL] First batch of KVM changes for 4.1

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
	gleb@kernel.org, kvm@vger.kernel.org,
	Ralf Baechle <ralf@linux-mips.org>,
	luto@kernel.org
Subject: Re: [GIT PULL] First batch of KVM changes for 4.1
Date: Wed, 22 Apr 2015 17:56:02 -0300	[thread overview]
Message-ID: <20150422205602.GA15317@amt.cnet> (raw)
In-Reply-To: <55353058.2000008@redhat.com>

On Mon, Apr 20, 2015 at 06:59:04PM +0200, Paolo Bonzini wrote:
> 
> 
> On 17/04/2015 22:18, Marcelo Tosatti wrote:
> > The bug which this is fixing is very rare, have no memory of a report.
> > 
> > In fact, its even difficult to create a synthetic reproducer.
> 
> But then why was the task migration notifier even in Jeremy's original
> code for Xen? 

To cover for the vcpu1 -> vcpu2 -> vcpu1 case, i believe.

> Was it supposed to work even on non-synchronized TSC?

Yes it is supposed to work on non-synchronized TSC.

> If that's the case, then it could be reverted indeed; but then why did
> you commit this patch to 4.1? 

Because it fixes the problem Andy reported (see Subject: KVM: x86: fix
kvmclock write race (v2) on kvm@). As long as you have Radim's
fix on top.

> Did you think of something that would
> cause the seqcount-like protocol to fail, and that turned out not to be
> the case later?  I was only following the mailing list sparsely in March.

No.