Re: [Xen-devel] Re: BUG: unable to handle kernel paging request at ffff8803bb6ad000

[Ian Campbell <ijc@xxxxxxxxxxxxxx>]

On Tue, 2011-10-11 at 08:07 +0100, Jan Beulich wrote:
> >>> On 10.10.11 at 18:49, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>> wrote:
> > On Sat, Oct 08, 2011 at 10:13:14AM +0400, rush wrote:
> >> OK, I tried it again, but Oops didn't gone.
> > .. snip..
> >>         echo    'Loading Xen 4.0-amd64 ...'
> >>         multiboot       /boot/xen-4.0-amd64.gz placeholder xsave=0
> > .. snip..
> >> Was it right?
> > 
> > Yup. I think.. this is a bit embarrassing. It took a bit of time for Intel
> > folks to get the xsave part right and I remember seeing this error about a
> > year ago with xsave on a Dell Optiplex 780. Hence I wonder if the fixes that
> > ultimately went in 4.1.1 did not get ported over to 4.0 and you are just
> > hitting that.
> > 
> > Can I ask you to do one more thing? Can you upgrade to the xen-4.1.1 in
> > the testing and try with the xsave (or without) and see if it works?
> > 
> > <holds his fingers hoping it is the xsave feature>
> 
> Are both of you certain this isn't the problem of the kernel only
> looking at the xsaveopt feature flag (implying that this means
> xsave is also available)? I found it necessary to force-clear that
> flag in the kernel when OSXSAVE is not set (by calling
> x86_xsave_setup() when !cpu_has_xsave, which in turn was
> modified to look at X86_FEATURE_OSXSAVE rather than
> X86_FEATURE_XSAVE under Xen - all of which I'm afraid would
> need to be done differently in pv-ops).

That all sounds familiar... In mainline we have (in
xen_init_cpuid_mask):

        ...
        xsave_mask =
                (1 << (X86_FEATURE_XSAVE % 32)) |
                (1 << (X86_FEATURE_OSXSAVE % 32));

        /* Xen will set CR4.OSXSAVE if supported and not disabled by force */
        if ((cx & xsave_mask) != xsave_mask)
                cpuid_leaf1_ecx_mask &= ~xsave_mask; /* disable XSAVE & OSXSAVE 
*/

Which I think implements something similar to what you describe? IOW
unless both XSAVE and OSXSAVE are available both are forcibly disabled.

While grepping I noticed that the kernel command line parameter to
disable xsave appears to be "noxsave" rather than "xsave=0", Rush is
that something you could try? (GRUB_CMDLINE_LINUX is the place to add
it)

Ian.

> If it is, the problem could be worked around by *en*abling xsave
> in Xen (which is off by default prior to 4.2), assuming none of the
> incomplete functionality would cause other headaches.
> 
> But yes, the CPUID handling code in 4.1.1 should properly hide
> XSAVEOPT when XSAVE is disabled, so just using this version
> ought to also get things going.
> 
> Jan
> 
> 
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-- 
Ian Campbell
Current Noise: Zyklon - Hammer Revelation

The ultimate game show will be the one where somebody gets killed at the end.
                -- Chuck Barris, creator of "The Gong Show"


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