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

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Message-ID: <20120911113715.GA28221@phenom.dumpdata.com>
Date:	Tue, 11 Sep 2012 07:37:15 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Matt Wilson <msw@...zon.com>, jforbes@...hat.com,
	kernel-maint@...hat.com
Cc:	"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 Mon, Sep 10, 2012 at 07:40:47PM -0700, Matt Wilson wrote:
> On Fri, Sep 07, 2012 at 11:00:22AM -0500, Justin M. Forbes wrote:
> > On Fri, 2012-09-07 at 16:44 +0100, Jan Beulich wrote:
> > > 
> > > All of this still doesn't provide evidence that a plain upstream
> > > kernel is actually having any problems in the first place. Further,
> > > if you say EC2 has a crippled hypervisor patch - is that patch
> > > available for looking at somewhere?
> > 
> > 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 

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