linux-kernel - Re: [Xen-devel] [PATCH/RFC] Fix xsave bug on older Xen hypervisors

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Message-ID: <CA+5PVA48UY6ckcKF3fMVvGk4rDtaZucff82gshyLkVgZEpppcw@mail.gmail.com>
Date:	Tue, 11 Sep 2012 09:06:51 -0400
From:	Josh Boyer <jwboyer@...il.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Matt Wilson <msw@...zon.com>, jforbes@...hat.com,
	"Justin M. Forbes" <jmforbes@...uxtx.org>,
	Jan Beulich <JBeulich@...e.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	xen-devel@...ts.xen.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH/RFC] Fix xsave bug on older Xen hypervisors

On Tue, Sep 11, 2012 at 7:37 AM, Konrad Rzeszutek Wilk
<konrad.wilk@...cle.com> wrote:
> On Mon, Sep 10, 2012 at 07:40:47PM -0700, Matt Wilson wrote:
>> > Yes, I can verify that a plain upstream kernel has problems in the first
>> > place, which is why we are carrying a patch to simply disable xsave all
>> > together in the pv guest.
>> > EC2 is not carrying a patch to cripple the hypervisor, there was an old
>> > xen bug that makes all this fail.  The correct fix for that bug is to
>> > patch the hypervisor, but they have not done so. Upstream xen has had
>> > the fix for quite some time, but that doesn't change the fact that a lot
>> > of xen guest usage these days is on EC2.  This is no different than
>> > putting in a quirk to work around a firmware bug in common use.
>>
>> I've done some testing and have results that indicate otherwise. The
>> out-of-tree xen_write_cr4() patch is not needed as of 2.6.39. I tested
>> 3.2.21 on a machine that has XSAVE capabilities:
>>
>> [ec2-user@...10-160-18-80 ~]$ cpuid -1 -i | grep -i xsave/xstor
>>       XSAVE/XSTOR states                      = true
>>       OS-enabled XSAVE/XSTOR                  = false
>>
>> on an older hypervisor build:
>>
>> [ec2-user@...10-160-18-80 ~]$ cat /sys/hypervisor/version/major
>> 3
>> [ec2-user@...10-160-18-80 ~]$ cat /sys/hypervisor/version/minor
>> 0
>>
>> and it boots without a problem. This patch correctly detects that the
>> hypervisor supports XSAVE by testing for OSXSAVE:
>>
>> commit 947ccf9c3c30307b774af3666ee74fcd9f47f646
>> Author:     Shan Haitao <haitao.shan@...el.com>
>> AuthorDate: Tue Nov 9 11:43:36 2010 -0800
>> Commit:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>> CommitDate: Wed Apr 6 08:31:13 2011 -0400
>>
>>     xen: Allow PV-OPS kernel to detect whether XSAVE is supported
>>
>>     Xen fails to mask XSAVE from the cpuid feature, despite not historically
>>     supporting guest use of XSAVE.  However, now that XSAVE support has been
>>     added to Xen, we need to reliably detect its presence.
>>
>>     The most reliable way to do this is to look at the OSXSAVE feature in
>>     cpuid which is set iff the OS (Xen, in this case), has set
>>     CR4.OSXSAVE.
>>
>> Matt
>
> Hey Matt,
>
> Thank you for testing. CC-ing some of the Fedora folks so they are aware that they
> can ditch the: fix_xen_guest_on_old_EC2.patch

Ditched.  Thanks Matt and Konrad.

(BTW, kernel-team@...oraproject.org is the easiest way to get things to
the fedora kernel team.)

josh
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