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

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Message-Id: <4FC649790200007800086E51@nat28.tlf.novell.com>
Date:	Wed, 30 May 2012 15:23:21 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andre Przywara" <andre.przywara@....com>
Cc:	"Jacob Shin" <Jacob.Shin@....com>, <mingo@...e.hu>,
	<jeremy@...p.org>, <tglx@...utronix.de>,
	<xen-devel@...ts.xensource.com>, <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH] x86/amd: fix crash as Xen Dom0 on AMD
 Trinity systems

>>> On 30.05.12 at 16:02, Andre Przywara <andre.przywara@....com> wrote:
> On 05/30/2012 03:33 PM, Jan Beulich wrote:
>> Further, I can't see how checking_wrmsrl() is being paravirtualized
>> any better than wrmsrl_amd_safe() - both have nothing but an
>> exception handling fixup attached to the wrmsr invocation. Care
>> to point out what actual crash it is that was seen?
> 
> AFAIK, the difference is between the "l" and the regs version for 
> rd/wrmsr. We have a patch already here to fix this. Will send it out 
> soon. Jacob, can you comment on this?

I see - the Xen code blindly overwrites pv_cpu_ops, despite not
having initialized all members. That's an obvious oversight of the
patch that introduced the _regs variants.

Plus having secondary instances of things like rdmsrl_amd_safe()
in asm/paravirt.h seems pretty strange an approach (which was
why initially I didn't spot how a crash could happen there) - only
the lowest level functions should get re-implemented here.

>> Finally, I would question whether re-enabling the topology
>> extensions under Xen shouldn't be skipped altogether, perhaps
>> even on Dom0 (as the hypervisor is controlling this MSR, but in
>> any case on DomU - the hypervisor won't allow (read: ignore,
>> not fault on) the write anyway (and will log a message for each
>> (v)CPU that attempts this).
> 
> This is probably right. Let me think about this.

I'll submit a respective hypervisor side patch soonish.

Jan

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