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Re: [Xen-devel] SSE instruction emulation issues

Il 15/07/2015 13:35, Jan Beulich ha scritto:
On 15.07.15 at 13:13, <fabio.fantoni@xxxxxxx> wrote:
Il 10/07/2015 14:16, Jan Beulich ha scritto:
On 10.07.15 at 14:00, <andrew.cooper3@xxxxxxxxxx> wrote:
On 09/07/15 20:32, Zhi Wang wrote:
      We found that MOVD instruction are used by some windows driver
during developing XenGT, and also we found this one:

(XEN) MMIO emulation failed: d7v1 64bit @ 0010:fffff8000294e273 -> 66
0f e7 00 48 83 c0 10 45 3
b cb 73 f0 45 85 c9
     0:    66 0f e7 00              movntdq %xmm0,(%rax)
     4:    48 83 c0 10              add    $0x10,%rax
     8:    45 3b cb                 cmp    %r11d,%r9d
     b:    73 f0                    jae    0xfffffffffffffffd
     d:    45 85 c9                 test   %r9d,%r9d

The x86 instruction emulator does appear to have a decode for this
instruction.  This failure suggests that the implementation is buggy.

To start with diagnosing, add a test case to
Considering that we already test MOVDQU, the emulation of which
shares code with MOVNTDQ (which only differs in aspects not of
interest to the emulator) I'm not sure this will turn up anything
interesting. Perhaps an even easier step would be to simply run
the emulator test on the machine where the issue is seen? We're
playing some prefix byte tricks there... Otoh failure to execute
the constructed instruction would bring down the hypervisor.
I also have a problem with mmio as I already reported many times but I
And to be honest, I don't see the value in re-stating this every once
in a while without providing any new information.

don't know if it is the same as the one reported by the intel developer
about xengt, I have it in linux hvm domUs with qxl.
Looks different - their's was about MOVD (which we clearly don't
support right now) while yours looks to be about MOVAPS.

Today with the latest xen update from git staging (with the addiction of
the xengt patch that add support of emulating SSE2 instruction MOVD) I
had a different domU's Xorg backtrace containing also a "error: Cannot
access memory at address":
Sadly a gdb backtrace is nothing I can see use extract useful information
from. Iirc Paul had already asked you to instrument the involved code
paths (considering that the x86 insn emulator supports MOVAPS as used
by the failing code) to figure out where in the whole involved stack the
failure actually originates.


Thanks for your reply, as I wrote the other times I don't know a better debug method about particular things like this (x86 instructions emulation) and I'm asking what I should do. If you mean to look at the code involved, search the part about the problem, think how can go wrong or unexpected, add debug output if needed, try quick changes to it ecc... I can do it with simpler software and I did something similar with libxl but I don't know how to do the same for code like xen/arch/x86/x86_emulate/x86_emulate.c. I already took a look at it but I didn't find "MOVAPS" in comments like many others. If the problem is located in something like libxl where there are instructions that I know or that are intuitive I can imagine what the software is supposed to do and I can do quick targeted tests or changes, but on thing like x86 emulation I can't (or at least not before knowing all instructions and essential data about it). Is this what you mean and is that the only way to collect useful data or to solve the problem? If so, I suppose that for any change in xen/arch/x86/x86_emulate and similar I can't simply make the change, do a make, make install and test it immediatly like libxl/xl but I have to rebuild full xen, install it and reboot dom0, is it right? Can you post a link with a quick reference about x86 emulation and/or instruction sets like sse2 which can help me learn what to do or an extensive knowledge on the subject is required in this case? What kind of logging instruction for debug can I use? Are they visible with xl dmesg or I must do something different and more specific in this case?

Thanks for any reply and sorry for my bad english.

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