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Re: [Xen-devel] Kernel panic with 2.6.32-30 under network activity

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] Kernel panic with 2.6.32-30 under network activity
From: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Date: Wed, 16 Mar 2011 10:11:08 +0000
Cc: Olivier Hanesse <olivier.hanesse@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Xen Users <xen-users@xxxxxxxxxxxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
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On Wed, 2011-03-16 at 09:34 +0000, Jan Beulich wrote:
> >>> On 16.03.11 at 04:20, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>> wrote:
> > On Thu, Mar 10, 2011 at 12:25:55PM +0100, Olivier Hanesse wrote:
> >> [469390.126691] alignment check: 0000 [#1] SMP
> > 
> > aligment check? Was there anything else in the log before this? Was there
> > anything in the Dom0 log?
> This together with
> >> [469390.126795] RSP: e02b:ffff88001ec3f9b8  EFLAGS: 00050286
> makes me wonder if either eflags got restored from a corrupted
> stack slot somewhere, or whether something in the kernel or one
> of the modules intentionally played with EFLAGS.AC.

Can a PV kernel running in ring-3 change AC? 

The Intel manual says "They should not be modified by application
programs" over a list including AC but the list also includes e.g. IOPL
and IF so I suspect it meant "can not" rather than "should not"? In
which case it can't happen by accident.

The hypervisor appears to clear the guest's EFLAGS.AC on context switch
to a guest and failsafe bounce but not in e.g. do_iret so it's not
entirely clear what his policy is... 


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