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Re: [Xen-devel] SSE instruction emulation issues

Il 15/07/2015 16:35, Wang, Zhi A ha scritto:
You can put the MMIO emulation failed output message in the email like what I 
did, that will help to cook a patch for instruction emulator. Only gdb log is 
not enough as xen-developer has to know the exact opcode. I also found that not 
all forms of one SSE instruction was supported, for example an instruction may 
support move data from xmm register to mem, or move from xmm register to xmm 
registers, maybe only one form is supported in the instruction emulator.

Thanks for your reply, unfortunately don't give directly "MMIO emulation failed" but on "better" cases only software crash inside domUs, on other only qemu at 100% cpu and in domUs black screen ecc... There are also many other cases of "strange" low performance probably related to MMIO operations. I started to find it some years ago on hvm desktop domUs, derisory/hardly visible on older windows like xp or linux DE with fewer effects and lower screen resolution of client used and increasing using newer system, DE and bigger resolutions. Using only thing like rdp inside domU's S.O. the problem is minimized, using client for connecting directly to domUs like spice is increased. Seems increased also using rdp but using softwares that use "more graphic operations" (even if not 3D). Trying similar thing on kvm instead using same qemu upstream version similar domU configuration ecc.. is better. About more specific problem with qxl vga on xen was unable to works at all until 2 memory fixes in libxl/qemu did by anthony perard 3-4 years ago is I remember good, after on windows domUs was working but with very bad performance, after Jan beulich a fix about x86/hvm 2-3 years performances became decent and other x86/hvm changes in latest 1-2 years they made the performance closer to kvm for most of the vm windows. About linux domUs qxl seems correctly working on suse as dom0 and domUs as reported by a suse developer but I not found the specific suse workaround, may be also other cases where is working out-of-box. Xen is already better for many features and find/solves this problem probably make it very good also in all recent desktop necessity (except particular 3d acceleration that require phisical vga or similar).

About cases where I taken backtrace of domU's X crash I suppose I must find possible fails case without output in xl dmesg and add output to find what is the exact case, right? I should add gdprintk(XENLOG_INFO,"...\n"); in any fail_if() and generate_exception_if() that can be related to movaps for example based on backtrace, or is this wrong or stupid?

Thanks for any reply and sorry for my bad english.


-----Original Message-----
From: Fabio Fantoni [mailto:fabio.fantoni@xxxxxxx]
Sent: Wednesday, July 15, 2015 9:56 PM
To: Jan Beulich
Cc: Andrew Cooper; Paul Durrant; Wang, Zhi A; xen-devel
Subject: Re: 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
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 
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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