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    SubjectRe: [Xen-devel] [PATCH/RFC] Fix xsave bug on older Xen hypervisors
    >>> On 07.09.12 at 16:22, "Justin M. Forbes" <> wrote:
    > On Fri, Sep 07, 2012 at 03:02:29PM +0100, Jan Beulich wrote:
    >> >>> On 07.09.12 at 15:21, Stefan Bader <> wrote:
    >> > On 07.09.2012 14:33, Jan Beulich wrote:
    >> >>>>> On 07.09.12 at 13:40, Stefan Bader <> 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?


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      Last update: 2012-09-07 18:41    [W:0.027 / U:8.692 seconds]
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