[Bug 1922887] Re: STR in Thumb 32 decode problem

[Peter Maydell]

Patch sent to list: if you could test it against whatever your test case was 
that would be helpful.
20210408162402.5822-1-peter.maydell@linaro.org/">https://patchew.org/QEMU/20210408162402.5822-1-peter.maydell@linaro.org/

PS: out of interest, why/how were you checking should-UNDEF cases ?


** Changed in: qemu
       Status: New => In Progress

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https://bugs.launchpad.net/bugs/1922887

Title:
  STR in Thumb 32 decode problem

Status in QEMU:
  In Progress

Bug description:
  Hi

  It seems that QEMU does not have a proper check on the STR instruction
  in Thumb32 mode.

  Specifically, the machine code is 0xf84f0ddd, which is 0b1111 1000 0100 1111 
0000 1101 1101 1101. 
  This is an STR (immediate, Thumb) instruction with a T4 encoding scheme.

  The symbols is

  Rn = 1111
  Rt = 0000
  P = 1
  U = 0
  W = 1

  The decode ASL is below:

  if P == ‘1’ && U == ‘1’ && W == ‘0’ then SEE STRT;
  if Rn == ‘1101’ && P == ‘1’ && U == ‘0’ && W == ‘1’ && imm8 == ‘00000100’ 
then SEE PUSH;
  if Rn == ‘1111’ || (P == ‘0’ && W == ‘0’) then UNDEFINED;
  t = UInt(Rt); n = UInt(Rn); imm32 = ZeroExtend(imm8, 32);
  index = (P == ‘1’); add = (U == ‘1’); wback = (W == ‘1’);
  if t == 15 || (wback && n == t) then UNPREDICTABLE;

  When Rn == 1111, it should be an undefined instruction, which should
  raise SEGILL signal. However, it seems that QEMU does not check this
  constraint, which should be a bug. Many thanks

  Regards
  Muhui

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