Re: [Qemu-devel] [Bug 1156313] [NEW] X86-64 flags handling broken

[Torbjorn Granlund <tg@gmplib.org>]

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Richard Henderson <rth@twiddle.net> writes:

  Patch at http://patchwork.ozlabs.org/patch/229139/
  
Thanks.  I can confirm that this fixes the bug triggered by my test case
(and yours).  However, the instability of Debian GNU/Linux x86_64 has
not improved.

The exact same Debian version (debian "testing") updated at the same
time runs well on hardware.

My qemu Debian system now got messed up, since I attempted an upgrade in
the buggy qemu, which segfaulted several times during the upgrade.  I
need to reinstall, and then rely on -snapshot.

There is a problem with denorms which is reproducible, but whether that
is a qemu bug, and whether it can actually cause the observed
instability, is questionable.  Here is a testcase for that problem:


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	.text
	.globl	main
	.type	main, @function
main:
	mov	$0x10000000000000, %rax		# DBL_MIN
	movq	%rax, %xmm0
	movabsq	$0x4000000000000000, %rax	# 2.0
	movq	%rax, %xmm1
	divsd	%xmm1, %xmm0
	xorpd	%xmm1, %xmm1

1:	ucomisd	%xmm0, %xmm1
	je	1b

	mov	$0, %eax
	ret

	.size	main, .-main

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It should terminate.  The observed buggy behaviour is that it hangs.

The instability problem can be observed at gmplib.org/devel/tm-date.html.
hwl-deb.gmplib.org is Debian under qemu with -cpu Haswell,+adx.

Not that the exact same qemu runs FreeBSD flawlessly (hwl.gmplib.org).
It is neither instable nor does it run the denorms testcase poorly.

I fully realise this is a hopeless bug report, but I am sure you can
reproduce it, since it is far from GMP specific.  After all apt-get
update; apt-get upgrade triggered it.  Debugging it will be a nightmare.

Qemu version: main git repo from less than a week ago + Richard ADX
patch.

-- 
Torbjörn