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Date: Sat, 6 Nov 2010 20:16:13 +0300 (MSK)
From: malc <av1474@comtv.ru>
Subject: Re: [Qemu-devel] Bug report about x86 'bt' insn
In-Reply-To: <86eiay344b.fsf@shell.gmplib.org>
Message-ID: <alpine.LNX.2.00.1011062015030.1406@linmac>
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To: Torbjorn Granlund <tg@gmplib.org>
Cc: qemu-devel@nongnu.org

On Sat, 6 Nov 2010, Torbjorn Granlund wrote:

> [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.

ZF is undefined according to AMD's 24594.pdf page 69.

> 
> (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.)
> 
> 

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