>>> 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)? 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/
