>>> On 07.09.12 at 16:22, "Justin M. Forbes" <jmfor...@linuxtx.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.ba...@canonical.com> wrote: >> > On 07.09.2012 14:33, Jan Beulich wrote: >> >>>>> On 07.09.12 at 13:40, Stefan Bader <stefan.ba...@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)? >> > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
