[Qemu-devel] [Bug 1810545] [NEW] [alpha] Strange exception address reported

* [Qemu-devel] [Bug 1810545] [NEW] [alpha] Strange exception address reported
@ 2019-01-04 18:21 Stefan Ring
  2019-01-04 19:05 ` [Qemu-devel] [Bug 1810545] " Peter Maydell
                   ` (4 more replies)
  0 siblings, 5 replies; 8+ messages in thread
From: Stefan Ring @ 2019-01-04 18:21 UTC (permalink / raw)
  To: qemu-devel

Public bug reported:

For some reason the SIGILL handler receives a different address under
qemu than it used to on real hardware. I don't know specifics about the
hardware used back then – it was some sort of 21264a somewhere between
600-800 MHz –, and I cannot say anything about the kernel as well, but I
know that it delivered the faulting address +4, while under qemu it
receives +8. I know because CACAO, an early Java JIT compiler extracts
the address from the SIGILL handler and inspects the code at the
faulting site, and it has substracted 4 from the handler address since
the dawn of time, and this used to produce the desired result on the
Alpha hardware. It actually ran on two different Alpha machines over the
years, and both behaved identically.

The handler looks like this:
void handler_sigill(int sig, siginfo_t *siginfo, void *_p)
{
  uintptr_t trap_address = (uintptr_t) (((ucontext_t*) _p)->uc_mcontext.sc_pc) - 4;
}

(paraphrasing, the actual code is here: https://bitbucket.org/cacaovm
/cacao-
staging/src/c8d3fbab864c3243f97629fcfa8d84ba71f38157/src/vm/jit/alpha/linux
/md-os.cpp?at=default&fileviewer=file-view-default#md-os.cpp-65)

I don't know much about the qemu source code and cannot say where this
is coming from at first glance. The gen_invalid function uses pc_next,
which sounds like the next instruction, not the next-to-next ;). In
theory it could actually be the kernel's fault, although I consider this
unlikely.

This is qemu-system-alpha with apparently the last Debian which existed
for Alpha (lenny). The kernel is 2.6.26-2-alpha-generic (Debian
2.6.26-29). Observed with qemu git 1b3e80082b, but I guess it is the
same with any version.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1810545

Title:
  [alpha] Strange exception address reported

Status in QEMU:
  New

Bug description:
  For some reason the SIGILL handler receives a different address under
  qemu than it used to on real hardware. I don't know specifics about
  the hardware used back then – it was some sort of 21264a somewhere
  between 600-800 MHz –, and I cannot say anything about the kernel as
  well, but I know that it delivered the faulting address +4, while
  under qemu it receives +8. I know because CACAO, an early Java JIT
  compiler extracts the address from the SIGILL handler and inspects the
  code at the faulting site, and it has substracted 4 from the handler
  address since the dawn of time, and this used to produce the desired
  result on the Alpha hardware. It actually ran on two different Alpha
  machines over the years, and both behaved identically.

  The handler looks like this:
  void handler_sigill(int sig, siginfo_t *siginfo, void *_p)
  {
    uintptr_t trap_address = (uintptr_t) (((ucontext_t*) _p)->uc_mcontext.sc_pc) - 4;
  }

  (paraphrasing, the actual code is here: https://bitbucket.org/cacaovm
  /cacao-
  staging/src/c8d3fbab864c3243f97629fcfa8d84ba71f38157/src/vm/jit/alpha/linux
  /md-os.cpp?at=default&fileviewer=file-view-default#md-os.cpp-65)

  I don't know much about the qemu source code and cannot say where this
  is coming from at first glance. The gen_invalid function uses pc_next,
  which sounds like the next instruction, not the next-to-next ;). In
  theory it could actually be the kernel's fault, although I consider
  this unlikely.

  This is qemu-system-alpha with apparently the last Debian which
  existed for Alpha (lenny). The kernel is 2.6.26-2-alpha-generic
  (Debian 2.6.26-29). Observed with qemu git 1b3e80082b, but I guess it
  is the same with any version.

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