linux-kernel - Re: [Xen-devel] [PATCH] x86 fixes for 3.3 impacting distros (v1).

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Message-Id: <4FEC9686020000780008C9BB@nat28.tlf.novell.com>
Date:	Thu, 28 Jun 2012 16:38:14 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	<cyclonusj@...il.com>, <rostedt@...dmis.org>,
	"Suresh B Siddha" <suresh.b.siddha@...el.com>,
	<marmarek@...isiblethingslab.com>, <x86@...nel.org>,
	<tglx@...utronix.de>, <xen-devel@...ts.xensource.com>,
	<mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	"Jason Garrett-Glaser" <jason@...4.com>
Subject: Re: [Xen-devel] [PATCH] x86 fixes for 3.3 impacting distros
 (v1).

>>> On 28.06.12 at 16:42, "H. Peter Anvin" <hpa@...or.com> wrote:
> On 06/28/2012 07:28 AM, Konrad Rzeszutek Wilk wrote:
>> I also chatted with the core Xen hypervisor folks about adding in the 
> context switch code
>> to alter the PAT layout - but they were not keen a about it - and I am not 
> sure how much
>> CPU cycles one loses by doing a wrmsr to the PAT register on every guest 
> context switch
>> (worst case when on has a pvops kernel and a old-style one - where the WC bit 
> would differ)?
>> 
> 
> And you're comparing that to a bunch of new pvops calls?  The discussion
> shouldn't even have started until you had ruled out this solution and
> had data to show it.

That's definitely not an option: Xen itself may be (and is, under
certain circumstances at least) using WC page table entries, so
we can't allow on-the-fly changes to the meaning of the various
indexes.

Jan

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