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

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Date:	Fri, 07 Sep 2012 15:02:29 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Stefan Bader" <stefan.bader@...onical.com>
Cc:	"Matt Wilson" <msw@...zon.com>, <xen-devel@...ts.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH/RFC] Fix xsave bug on older Xen
 hypervisors

>>> On 07.09.12 at 15:21, Stefan Bader <stefan.bader@...onical.com> wrote:
> On 07.09.2012 14:33, Jan Beulich wrote:
>>>>> On 07.09.12 at 13:40, Stefan Bader <stefan.bader@...onical.com> wrote:
>>> When writing unsupported flags into CR4 (for some time the
>>> xen_write_cr4 function would refuse to do anything at all)
>>> older Xen hypervisors (and patch can potentially be improved
>>> by finding out what older means in version numbers) would
>>> crash the guest.
>>>
>>> Since Amazon EC2 would at least in the past be affected by that,
>>> Fedora and Ubuntu were carrying a hack that would filter out
>>> X86_CR4_OSXSAVE before writing to CR4. This would affect any
>>> PV guest, even those running on a newer HV.
>>>
>>> And this recently caused trouble because some user-space was
>>> only partially checking (or maybe only looking at the cpuid
>>> bits) and then trying to use xsave even though the OS support
>>> was not set.
>>>
>>> So I came up with a patch that would
>>> - limit the work-around to certain Xen versions
>>> - prevent the write to CR4 by unsetting xsave and osxsave in
>>>   the cpuid bits
>>>
>>> Doing things that way may actually allow this to be acceptable
>>> upstream, so I am sending it around, now.
>>> It probably could be improved when knowing the exact version
>>> to test for but otherwise should allow to work around the guest
>>> crash while not preventing xsave on Xen 4.x and newer hosts.
>> 
>> Before considering a hack like this, I'd really like to see evidence
>> of the described behavior with an upstream kernel (i.e. not one
>> with that known broken hack patched in, which has never been
>> upstream afaict).
> 
> This is the reason I wrote that Fedora and Ubuntu were carrying it. It never 
> has
> been send upstream (the other version) because it would filter the CR4 write 
> for
> any PV guest regardless of host version.

But iirc that bad patch is a Linux side one (i.e. you're trying to fix
something upstream that isn't upstream)?

Jan

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