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

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Date:	Fri, 07 Sep 2012 16:44:32 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Justin M. Forbes" <jmforbes@...uxtx.org>
Cc:	"Matt Wilson" <msw@...zon.com>,
	"Stefan Bader" <stefan.bader@...onical.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 16:22, "Justin M. Forbes" <jmforbes@...uxtx.org> wrote:
> On Fri, Sep 07, 2012 at 03:02:29PM +0100, Jan Beulich wrote:
>> >>> 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)?
>> 
> Right, so the patch that this improves upon, and that Fedora and Ubuntu are
> currently carrying is not upstream because:
> 
> a) It's crap, it cripples upstream xen users, but doesn't impact RHEL xen
> users because xsave was never supported there.
> 
> b) The hypervisor was patched to make it unnecessary quite some time ago,
> and we hoped EC2 would eventually pick up that correct patch and we could
> drop the crap kernel patch.
> 
> Unfortunately this has not happened. We are at a point where EC2 really is
> a quirk that has to be worked around. Distros do not want to maintain
> a separate EC2 build of the kernel, so the easiest way is to cripple
> current upstream xen users.  This quirk is unfortunately the best possible
> solution.  Having it upstream also makes it possible for any user to build
> an upstream kernel that will run on EC2 without having to dig a random
> patch out of a vendor kernel.

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?

Jan

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