[Qemu-devel] Bug report about x86 'bt' insn

[Torbjorn Granlund <tg@gmplib.org>]

[This is a bug report, reported here since I was redirected to some
corporate site when attempting to follow the recommended bug report
practice.  If I am really required to register an account with a
corporation in order to report a qemu bug, and that bug reports are not
accepted here, then please accept my apology; I suggest that you simply
ignore this bug report.]

Bug report:

Qemu version: 0.13.0 (installed via FreeBSD's ports system).

Host: FreeBSD 8.1 amd64 (without any relevant kernel modules)

Guest environments where problem has been reproduced: (1) Debian GNU/Linux
5, (2) Debian GNU/kFreeBSD snapshot, (3) FreeBSD 8.1

Problem: The bt insn works incorrectly under qemu.  It incorrectly sets
the Z flag.  The bt insn should only affect the carry flag.  (This
causes the GMP library to work incorrectly.)

Test code snippet:

.text
	.globl	main
main:	mov	$1, %eax
	and	%eax, %eax
	bt	$0, %eax
	jz	1f
	xor	%eax, %eax
1:	ret

Assembly and execute.  Correct behaviour is to exit(0).  Under qemu it
it exits with value 1.

(I thought qemu wouldn't interfere with plain user mode instructions
when the host system can handle all the guest systems's instructions,
and that they would execute natively and at full speed.  This bug
suggests that buggy emulation is taking place.)

-- 
Torbjörn