linux-kernel - Re: [Xen-devel] [PATCH] x86/amd: fix crash as Xen Dom0 on AMD Trinity systems

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Date:	Wed, 30 May 2012 16:08:30 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Andre Przywara" <andre.przywara@....com>, <mingo@...e.hu>,
	<jeremy@...p.org>, <tglx@...utronix.de>,
	<xen-devel@...ts.xensource.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org#3.4+>
Subject: Re: [Xen-devel] [PATCH] x86/amd: fix crash as Xen Dom0 on AMD
 Trinity systems

>>> On 30.05.12 at 16:50, "H. Peter Anvin" <hpa@...or.com> wrote:
> On 05/30/2012 07:39 AM, Konrad Rzeszutek Wilk wrote:
>> On Wed, May 30, 2012 at 03:10:02PM +0200, Andre Przywara wrote:
>>> Because we are behind a family check before tweaking the topology
>>> bit, we can use the standard rd/wrmsr variants for the CPUID feature
>>> register.
>>> This fixes a crash when using the kernel as a Xen Dom0 on affected
>>> Trinity systems. The wrmsrl_amd_safe is not properly paravirtualized
>>> yet (this will be fixed in another patch).
>> 
>> So with a rdmsrl_amd_safe and wrmsrl_amd_safe being implemented in
>> the pv_cpu_ops - would this patch even be neccessary?
>> 
> 
> That is still bogus;  a better thing would be to implement the _regs
> interface.  Even better would be to trap and emulate rdmsr/wrmsr!

The crash is not on the wrmsr instruction, but on the paravirt
layer finding a NULL pointer in one of the methods. Xen does
trap and emulate (possibly just ignore) both instructions.

Jan

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