From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "Mihai Donțu" <mihai.dontu@gmail.com>, wogiz@openmailbox.org
Cc: xen-devel@lists.xen.org
Subject: Re: Bug in x86 instruction emulator?
Date: Wed, 6 Apr 2016 09:55:16 +0100 [thread overview]
Message-ID: <5704CEF4.2050308@citrix.com> (raw)
In-Reply-To: <20160406025735.1ffbb1a6@mdontu-l>
On 06/04/16 00:57, Mihai Donțu wrote:
> On Wed, 06 Apr 2016 01:38:32 +0200 wogiz@openmailbox.org 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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next prev parent reply other threads:[~2016-04-06 8:55 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-05 23:38 Bug in x86 instruction emulator? wogiz
2016-04-05 23:57 ` Mihai Donțu
2016-04-06 0:02 ` Mihai Donțu
2016-04-06 1:48 ` wogiz
2016-04-06 1:26 ` wogiz
2016-04-06 8:55 ` Andrew Cooper [this message]
2016-04-07 1:26 ` wogiz
2016-04-07 2:04 ` Jan Beulich
2016-04-08 1:43 ` wogiz
2016-04-15 17:33 ` wogiz
2016-04-15 17:44 ` Andrew Cooper
2016-04-16 4:06 ` wogiz
2016-05-04 16:02 ` Jan Beulich
2016-05-04 16:04 ` Wei Liu
2016-05-04 16:06 ` Andrew Cooper
2016-05-17 16:53 ` William Z.
2016-05-17 17:03 ` Andrew Cooper
2016-05-18 9:12 ` Jan Beulich
2016-05-20 16:44 ` William Z.
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