Re: xen on suse 9.3 and software raid

Re: xen on suse 9.3 and software raid

[Gerd Knorr]

[ adding xen-devel to Cc: ]

> Just loading the raid6 module in a xen domain makes my
> machine oops, log below.  I'd guess it's because the
> module attempts to use mmx within the kernel, which is
> a bit tricky because you have to save/restore userspace
> fp state for that.  Could be a corner case which simply
> doesn't work with xen.

Confirmed, the faulting instruction is a attempt to access
cr0, right here:

  static inline unsigned long raid6_get_fpu(void)
  {
          unsigned long cr0;

          preempt_disable();
          asm volatile("mov %%cr0,%0 ; clts" : "=r" (cr0));
          return cr0;
  }

(which is called by raid6_before_mmx, which in turn is called by
raid6_mmx1_gen_syndrome, both calls not visible in the trace
because they are inlined).

Ideas how to fix that?  Other than just disabling raid6?  Can
that code be replaced with xen hypercalls?  fsave & frestore
are also used btw.  It's all in drivers/md/raid6x86.h

  Gerd

-- 
-mm seems unusually stable at present.
	-- akpm about 2.6.12-rc3-mm3

Re: Re: [Xen-users] xen on suse 9.3 and software raid

[Keir Fraser]

On 12 May 2005, at 15:28, Gerd Knorr wrote:

> (which is called by raid6_before_mmx, which in turn is called by
> raid6_mmx1_gen_syndrome, both calls not visible in the trace
> because they are inlined).
>
> Ideas how to fix that?  Other than just disabling raid6?  Can
> that code be replaced with xen hypercalls?  fsave & frestore
> are also used btw.  It's all in drivers/md/raid6x86.h

This is a known issue with Xen 2.0, already fixed in unstable (by 
emulating the privileged instructions).

There are three options here:
  1. Live with it. Disable raid6 in Xen 2.0 and wait for upgrade to 
forthcoming Xen 3.0.
  2. Backport some of the privileged-instruction emulation to Xen 2.0.
  3. Patch the raid6x86.h for Xen 2.0 to do hypercalls rather than priv 
instructions.

I think either options 1 or 2 would be best. Backporting the relevant 
bits of the instruction decoder and emulator from the unstable 
repository would not be very difficult, if this is a big show-stopper 
for anyone.

  -- Keir