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

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Date:	Thu, 23 Apr 2015 11:13:23 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Gleb Natapov <gleb@...nel.org>, kvm list <kvm@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Andrew Lutomirski <luto@...nel.org>
Subject: Re: [GIT PULL] First batch of KVM changes for 4.1



On 22/04/2015 23:21, Marcelo Tosatti wrote:
> On Mon, Apr 20, 2015 at 01:27:58PM -0700, Andy Lutomirski wrote:
>> On Mon, Apr 20, 2015 at 9:59 AM, Paolo Bonzini <pbonzini@...hat.com> 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?  Was it supposed to work even 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?  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.
>>
>> I don't think anyone ever tried that hard to test this stuff.  There
>> was an infinte loop that Firefox was triggering as a KVM guest
>> somewhat reliably until a couple months ago in the same vdso code.  :(
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1174664

That was the missing volatile in an asm.  Older compilers didn't catch
it. :(

Paolo
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