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RE: [Xen-devel] wget and Zope crashes on post-2.0.6 -testing

To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>, "Osma Suominen" <osma.suominen@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] wget and Zope crashes on post-2.0.6 -testing
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Wed, 8 Jun 2005 18:58:51 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Kip Macy <kip.macy@xxxxxxxxx>, Kurt Garloff <garloff@xxxxxxx>, Rich Persaud <rich@xxxxxxxxxxxxx>, Gerd Knorr <kraxel@xxxxxxx>
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Thread-index: AcVsUTxGpZ1TMYJOSmyjMikYjBKa6gAAEcaQ
Thread-topic: [Xen-devel] wget and Zope crashes on post-2.0.6 -testing
> On 3 Jun 2005, at 10:04, Osma Suominen wrote:
> > When you've had wget crash, you can try some of the other tests in
> > http://thread.gmane.org/gmane.comp.emulators.xen.devel/10628
> >
> > Since this happens on a random PC with the demo CD, I'll 
> bet that this 
> > is not some obscure problem with the specific hardware or software 
> > installation but a real bug in Xen.
> This bug should now be fixed in our xen-2.0.testing.bk repository.

This deserves a bit more explanation, as it probably effects all vendor
kernels based on Xen 2.0 (SuSE 9.3 Pro, Debian, demo CD, Gentoo, etc.)
It does *not* effect the kernel we ship in our 2.0 source and binary tar
balls, which is why its taken so long to pin down. It does *not* effect
the unstable branch.

The reason the bug is not present in our kernels is due to the kernel
config: we enable CONFIG_MD_RAID5=y in our config which hides the bug,
whereas most distros have this as a module.

The root cause of the bug is that during the boot sequence Linux tests
to see whether the processor has the fdiv bug. This involves doing some
floating point opertions. Unfortunately, they are not wrapped in the
kernel_fpu_begin()/end() calls that normally surround use of fp in the
kernel. Native linux gets away with this because it happens so early in
the boot process that no-one else can be using the fpu. However, on Xen
this gets us into a bad state, which will come back to haunt us much
later on, resulting in fpu state corruption in user processes. The fix
in 2.0-testing is simply to 'wrap' the fdiv test.

The reason the bug is not present on unstable is that the fpu code had
already been rejigged so that we were immune to this kind of problem as
it had been identified as a potential fragility.

Since this bug hadn't been widely reported we probably won't rush to
release a 2.0.6a demo CD, but vendor kernel maintainers should
definitely pick up the fix.


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