Re: [Xen-devel] Xen 4.3 development update RC2 imminent

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 22/05/13 17:30, Pasi Kärkkäinen wrote:
> On Wed, May 22, 2013 at 04:05:27PM +0100, George Dunlap wrote:
> > > > > > The emulator in the hypervisor can handle simple SSE instructions
> > > > > > like the above quite well. It's not immediately clear to me why
> > > > > > hvmemul_do_io() would need to limit the size to no more than a
> > > > > > long's width. Perhaps the data passing to the device model may
> > > > > > need adjustment to accommodate wider entities...
> > > > > Hmm, but the code seems to indicate that the DM can handle wider
> > > > > entities, by "reading all ones":
> > > > >
> > > > >          if ( dir == IOREQ_READ )
> > > > >              memset(p_data, ~0, size);
> > > > >
> > > > > Anthony, do you want to try making that size check one size bigger
> > > > > (e.g., allow it to be 16 or 32)?
> > > > No, that obviously won't work, because of the line just following:
> > > >
> > > >     if ( (p_data != NULL) && (dir == IOREQ_WRITE) )
> > > >     {
> > > >         memcpy(&value, p_data, size);
> > > >         p_data = NULL;
> > > >     }
> > > >
> > > >
> > > > value is of size "long", so this won't work.
> > > >
> > > > -George
> > > Thanks for help to solve this problem.
> > > Are there news about?
> > >
> > > Probably this is a stupid question: is this patch related to that
> > > problem?
> > > http://lists.xen.org/archives/html/xen-devel/2013-05/msg02142.html
> > No, I'm afraid that has nothing to do with this issue.  I've only
> > looked briefly at it, but it appears that the interface between Xen
> > and qemu is limited to MMIO accesses of 8 bytes; changing that
> > interface is not something we can really do while we're in the
> > middle of doing a release.
> >
> > The only work-around that would be suitable for 4.3 would be if we
> > could find an option to tell the X server not to execute SSE
> > instructions.  If there is no such work-around, then I'm afraid
> > we're going to have to disable the interface for 4.3.  We'll put it
> > on the list of work items for 4.4.
> Hmm, for testing, can we use cpuid to mask out SSE,
> and then try qxl ?
That had occurred to me -- Andrew / Jan, do you know which flag might 
disable this particular instruction?
I guess we could try just disabling all the SSE instructions.

Fabio: Can you do the following:
* On your host, do "cat /proc/cpuinfo".  Under "flags" there will be a 
big list.  Look for all of the ones that have "sse" in them.
On my AMD box, that includes sse, sse2, ssse3, sse4_1, and sse4_2.

* In your xl.cfg, add a cpuid with each of those flags disabled.

On my box, it looks like this:

cpuid="host,sse=0,sse2=0,ssse3=0,sse4_1=0,sse4_2=0"

Then run your system with Anthony's patch and see if you still get the 
crash.
 -George

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