Re: xsave=0 workaround needed on 3.2 kernels with Xen 4.1 or Xen-unstable.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Mon, May 07, 2012 at 05:41:28PM -0700, AP wrote:
> On Mon, May 7, 2012 at 5:08 PM, Konrad Rzeszutek Wilk <
> konrad.wilk@oracle.com> wrote:
> >
> > > > No, this is specifically the wrong thing. From what we know so far
> > > > (i.e. the outcome of the above printing you added) the problem in
> > > > in the Dom0 kernel (in it never setting CR4.OSXSAVE prior to
> > > > attempting XSETBV). What your patch efectively does is take away
> > > > control from the guest kernels to control the (virtual) CR4 flag...
> > > >
> > > > > That allowed the system to boot successfully though I did see the
> > > > > following message:
> > > > > (XEN) domain.c:698:d0 Attempt to change CR4 flags 00042660 ->
> > > > > 00002660
> > > >
> > > > ... which is what this message is telling you.
> > > >
> > > > > Not sure if the above patch is right fix but I hope it was at least
> > > > > helpful in pointing at where the problem might be.
> > > > >
> > > > > BTW, I see the same invalid op issue with Xen 4.1.2 if I boot with
> > > > > xsave=1.
> > > >
> > > > Sure, as it's a kernel problem. It's the kernel that needs logging
> > > > added,
> > > > to find out why the CR4 write supposedly happening immediately
> > > > prior to the XSETBV (set_in_cr4(X86_CR4_OSXSAVE)) doesn't actually
> > > > happen, or doesn't set the flag. Perhaps something fishy going on
> > >
> > > xen_write_cr4 explicitly turns off X86_CR4_OSXSAVE.
> >
> > Where did you see that code? Looking at the Linus's tree this is what I
> > see
> >
> >
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=arch/x86/xen/enlighten.c;h=a8f8844b8d32690b8a189bc37d12cd3f286a81cd;hb=HEAD
> >
> > So who added that code? I am not seeing it in v3.0 either?
> 
> This is in the Ubuntu 11.10 kernel:
> http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-oneiric.git;a=blob;f=arch/x86/xen/enlighten.c;h=ce01f6d63507fd44288989ca0ba81a0f5bf04e3f;hb=HEAD#l812
> 
> After some digging around, it looks like this is an Ubuntu 11.10 only patch:
> http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-oneiric.git;a=commitdiff;h=3f3fba59aa5773836d94799d10b692f9b7ea16a0;hp=5e498fdb19f5b27699f063eb10040612b824160b


Ugh. There are looks to be a bug in Fedora as well: https://bugzilla.redhat.com/show_bug.cgi?id=801650 for this.