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[Bug 1668041] Re: x86 Floating point exceptions - incorrect support?

From: Thomas Huth
Subject: [Bug 1668041] Re: x86 Floating point exceptions - incorrect support?
Date: Sun, 08 Nov 2020 09:14:48 -0000

The QEMU project is currently considering to move its bug tracking to another 
system. For this we need to know which bugs are still valid and which could be 
closed already. Thus we are setting all older bugs to
"Incomplete" now.
If you still think this bug report here is valid, then please switch the state 
back to "New" within the next 60 days, otherwise this report will be marked as 
"Expired". Thank you and sorry for the inconvenience.

** Changed in: qemu
       Status: New => Incomplete

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  x86 Floating point exceptions - incorrect support?

Status in QEMU:

Bug description:
  It seems that qemu does not correctly emulate the x86 support for
  optionally causing a floating-point exception (#FP) when, for example,
  dividing by zero. Reports such as:


  suggest that setting the exception mask in the fpu cw or mxcsr (e.g.,
  using a function like feenableexcept() in the guest OS) does not
  generate floating point exceptions on divide by zero. The problem only
  happens on pure QEMU - when a QEMU/KVM combination is used, the actual
  hardware does the floating point work, and does throw the exception on
  divide by zero if so requested.

  Looking at the qemu (2.8.0) source code, it seems to me it really
  lacks support for generating fpu exceptions: For example,
  helper_fdiv() in target-i386/fpu_helper.c, when it notices the divisor
  is zero, seems to set the divide-by-zero exception bit, but doesn't
  seem to check whether it needs to trigger an exception (when the right
  bits on the x87 or SSE control words are enabled).

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