linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: "Jan Beulich" <JBeulich@suse.com>
To: "Stefan Bader" <stefan.bader@canonical.com>
Cc: "Matt Wilson" <msw@amazon.com>, <xen-devel@lists.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [Xen-devel] [PATCH/RFC] Fix xsave bug on older Xen hypervisors
Date: Fri, 07 Sep 2012 15:02:29 +0100	[thread overview]
Message-ID: <504A1A950200007800099D4C@nat28.tlf.novell.com> (raw)
In-Reply-To: <5049F4E9.9050306@canonical.com>

>>> On 07.09.12 at 15:21, Stefan Bader <stefan.bader@canonical.com> wrote:
> On 07.09.2012 14:33, Jan Beulich wrote:
>>>>> On 07.09.12 at 13:40, Stefan Bader <stefan.bader@canonical.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


  reply	other threads:[~2012-09-07 14:01 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-09-07 11:40 [PATCH/RFC] Fix xsave bug on older Xen hypervisors Stefan Bader
2012-09-07 12:33 ` [Xen-devel] " Jan Beulich
2012-09-07 13:21   ` Stefan Bader
2012-09-07 14:02     ` Jan Beulich [this message]
2012-09-07 14:22       ` Justin M. Forbes
2012-09-07 14:54         ` Konrad Rzeszutek Wilk
2012-09-08 10:18           ` Paolo Bonzini
2012-09-10 22:36           ` Matt Wilson
2012-09-07 15:44         ` Jan Beulich
2012-09-07 15:47           ` Stefan Bader
2012-09-07 15:52             ` Jan Beulich
2012-09-07 15:48               ` Konrad Rzeszutek Wilk
2012-09-07 16:13               ` Stefan Bader
2012-09-08 10:28                 ` Paolo Bonzini
2012-09-07 15:54             ` Ian Campbell
2012-09-08 10:20             ` Paolo Bonzini
2012-09-07 16:00           ` Justin M. Forbes
2012-09-11  2:40             ` Matt Wilson
2012-09-11 11:37               ` Konrad Rzeszutek Wilk
2012-09-11 13:06                 ` Josh Boyer
2012-09-07 13:47 ` Konrad Rzeszutek Wilk
2012-09-11  1:17   ` Matt Wilson

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=504A1A950200007800099D4C@nat28.tlf.novell.com \
    --to=jbeulich@suse.com \
    --cc=konrad.wilk@oracle.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=msw@amazon.com \
    --cc=stefan.bader@canonical.com \
    --cc=xen-devel@lists.xen.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).