[Bug 1877794] Re: Constant Folding on 64-bit Subtraction causes SIGILL on linux-user glxgears ppc64le to x86_64 by way of generating bad shift instruction with c=-1

[Laurent Vivier]

QEMU has been selected to have a GSoC intern to work on new ioctl():
https://wiki.qemu.org/Google_Summer_of_Code_2020#Extend_linux-user_syscalls_and_ioctls

Perhaps he can help you by working on DRM ioctl()?
Perhaps you can send an RFC series to the QEMU mailing list?

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https://bugs.launchpad.net/bugs/1877794

Title:
  Constant Folding on 64-bit Subtraction causes SIGILL on linux-user
  glxgears ppc64le to x86_64 by way of generating bad shift instruction
  with c=-1

Status in QEMU:
  New

Bug description:
  Hello, I've been recently working on my own little branch of QEMU
  implementing the drm IOCTLs, when I discovered that glxgears seems to
  crash in GLXSwapBuffers(); with a SIGILL. I investigated this for
  about 2 weeks, manually trying to trace the call stack, only to find
  that we seemingly crash in a bad shift instruction. Originally
  intended to be an shr_i64 generated to an RLDICL, we end up with an
  all ones(-1) c value, which gets thrown to the macro for generating
  the MB, and replaces the instruction with mostly ones. This new
  instruction, FFFFFFE0 is invalid on ppc64le, and crashes in a host
  SIGILL in codegen_buffer. I tried to see if the output of translate.c
  had this bad instruction, but all I got were two (shr eax, cl)
  instructions, and upon creating a test program with shr (eax, cl) in
  it, nothing happened. Then figuring that there was nothing actually
  wrong with the instruction in the first place, I turned my eye to the
  optimizer, and completely disabled constant folding for arithmetic
  instructions.  This seemed to actually resolve the issue, and then I
  slowly enabled constant folding again on various instructions only to
  find that enabling not on the shifts, but on subtraction seemed to
  cause the bug to reappear. I am bewildered and frankly at this point
  I'm not sure I have a chance in hell of figuring out what causes it,
  so I'm throwing it here.

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