linux-kernel - Re: [PATCH] mce: use safe MSR accesses

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Message-ID: <5501CAC7.6090306@amd.com>
Date:	Thu, 12 Mar 2015 12:20:07 -0500
From:	Joel Schopp <joel.schopp@....com>
To:	"Luck, Tony" <tony.luck@...el.com>,
	"jesse.larrew@....com" <jesse.larrew@....com>,
	"x86@...nel.org" <x86@...nel.org>
CC:	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mce: use safe MSR accesses



On 03/11/2015 05:47 PM, Luck, Tony wrote:
>> When running as a guest under kvm, it's possible that the MSR
>> being accessed may not be implemented. All MSR accesses should
>> be prepared to handle exceptions.
> Isn't that a KVM bug?  The code here first checks family/model before accessing the MSR:
>
>                  if (c->x86 == 0x15 &&
>                      (c->x86_model >= 0x10 && c->x86_model <= 0x1f)) {
>
> If kvm tells the guest that it is running on one of these models, shouldn't it provide
> complete coverage for that model?
These MSRs don't make sense in guest mode.  The real question is if we
fix that in KVM, here, or both.  I'm a fan of fixing it in both places. 
Xen's behavior is to return a value of 0 if the guest tries to access
these, that seems like a reasonable thing to do in KVM as well.  I am
volunteering myself to write that patch for KVM, but I would encourage
accepting an updated version of this patch as well.
>
> If that isn't possible - then you should still do more than just s/rdmsrl/rdmsrl_safe/ ... like
> check the return value to see whether you got an exception .. and thus should skip past
> code that uses the "val" that you thought you read from the non-existent MSR.
Initializing val to 0 where it is declared should have the desired effect.
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