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

From: Josh Boyer
Date: Tue Sep 11 2012 - 09:06:44 EST

On Tue, Sep 11, 2012 at 7:37 AM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> 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@ip-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@ip-10-160-18-80 ~]$ cat /sys/hypervisor/version/major
>> 3
>> [ec2-user@ip-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@xxxxxxxxx>
>> AuthorDate: Tue Nov 9 11:43:36 2010 -0800
>> Commit: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>> 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
>> 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@xxxxxxxxxxxxxxxxx is the easiest way to get things to
the fedora kernel team.)

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