Re: Xen 4.3 development update RC2 imminent

[George Dunlap <george.dunlap@eu.citrix.com>]

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