Re: [Xen-devel] Bug in x86 instruction emulator?

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/04/16 00:57, Mihai Donțu wrote:
> On Wed, 06 Apr 2016 01:38:32 +0200 wogiz@xxxxxxxxxxxxxxx wrote:
>> I'm running Xen 4.6.1 with Alpine Linux 3.3.3 in dom0. In a HVM domU 
>> with vga="qxl", Xorg will segfault instantly if tried started. Multiple 
>> Linux distros have been tested and Xorg segfaults in all.
>>
>> Attached are a full backtrace from domU generated by Xorg, and a 
>> assembler dump of function 'sse2_blt'.
>>
>> According to Xen IRC channel, the cause could be a bug in the x86 
>> instruction emulator related to SSE.
> I don't believe the x86 emulator is complete wrt the SSE instruction
> set. But I do wonder why, in your case, these instructions need
> emulation at all. Unless touching the video RAM requires emulation. Can
> you try using a different video driver? I see xorg picked up qxl, maybe
> try vesa?
>

Now I think about it, even dirty VRAM tracking shouldn't actually
emulate the instructions.

Can you grab the full register state at the point of Xorgs crash?  `info
regs` in gdb?

The instruction in use, `movaps` is specified to fault if the memory
operand isn't aligned on a 16byte boundary.  Therefore, if %rax in this
case isn't a multiple of 16, this is a code generation bug, rather than
an emulation bug.

~Andrew

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