Re: Xen 4.3 development update RC2 imminent

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

On 05/21/2013 05:13 PM, George Dunlap wrote:
> On 05/21/2013 03:55 PM, Jan Beulich wrote:
>>>>> On 21.05.13 at 16:31, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>> On 21/05/13 15:06, Anthony PERARD wrote:
>>>> But then, once this applied, qxl is still not able to start. Xorg crash
>>>> (in the guest), and here is why:
>>>>
>>>> (XEN) emulate.c:88:d18 bad mmio size 16
>>>> (XEN) io.c:201:d18 MMIO emulation failed @ 0033:7fd2de390430: f3 0f 6f
>>>> 19 41 83 e8 403
>>>> (XEN) emulate.c:88:d18 bad mmio size 16
>>>> (XEN) io.c:201:d18 MMIO emulation failed @ 0033:7fd2de390430: f3 0f 6f
>>>> 19 41 83 e8 403
>>>
>>> Disassembly of section .data:
>>>
>>> 0000000000000000 <.data>:
>>>     0:    f3 0f 6f 19              movdqu (%rcx),%xmm3
>>>
>>> Xen does not support emulating SSE instructions.  We have sporadically
>>> seen similar errors from Windows guests.  The best guess I have managed
>>> to get so far is that %rcx is a pointer to something which Xen thinks is
>>> an MMIO page.
>>>
>>> In this case, it looks like X is copying from MMIO into an xmm register,
>>> scraping the framebuffer perhaps?  In the windows failure, it was the
>>> pagescrub trying to zero ram, which clearly indicated something wonky in
>>> the combined idea of the memory map.
>>>
>>> If Spice is doing something valid and sensible, then Xen will likely
>>> need extending to be able to emulate SSE instructions.
>>
>> 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