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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.10.11 at 10:02, Ian Campbell <ijc@xxxxxxxxxxxxxx> wrote:
> 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.

Apart from the need to disable XSAVEOPT, yes.

> 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)

Or "noxsaveopt" (if that's the problem, i.e. Rush's CPUs have that
capability).

Jan

> 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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>> Xen-devel@xxxxxxxxxxxxxxxxxxx 
>> http://lists.xensource.com/xen-devel 
> 
> -- 
> 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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