linux-kernel - Re: [PATCH] xen: fix logical error in tlb flushing

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Message-Id: <504723A70200007800098A79@nat28.tlf.novell.com>
Date:	Wed, 05 Sep 2012 09:04:23 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Alex Shi" <alex.shi@...el.com>
Cc:	"Yongjie Ren" <yongjie.ren@...el.com>, <tglx@...utronix.de>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	<mingo@...hat.com>, <linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] xen: fix logical error in tlb flushing

>>> On 05.09.12 at 07:34, Alex Shi <alex.shi@...el.com> wrote:
> On 08/25/2012 03:45 AM, Jan Beulich wrote:
>>>>> On 24.08.12 at 20:17, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
>>> How can I reproduce this
>>
>> I don't know, I spotted this while looking at the code.
> 
> Again, since the old buggy code doesn't cause trouble in PV guest, guess
> the hypercall for MMUEXT_INVLPG_MULTI was translated or treated as
> MMUEXT_TLB_FLUSH_MULTI. If so, believe correct this will bring a big
> performance benefit.

It's not clear to me what was buggy with the code prior to your
change. And no, there's no magic widening of the scope of these
MMU operations - if you ask the hypervisor for a single page
invalidation, that's what it's going to do. But of course, there
are cases where extra (full) invalidations need to be done
without a guest asking for them. But that's nothing a guest can
validly make itself dependent upon.

Jan

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