linux-kernel - Re: [PATCH] x86: don't rely on VMWare emulating PAT MSR correctly

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Date:	Wed, 17 Dec 2014 05:52:59 +0100
From:	Juergen Gross <jgross@...e.com>
To:	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
	akataria@...are.com, jongman.heo@...sung.com
Subject: Re: [PATCH] x86: don't rely on VMWare emulating PAT MSR correctly

On 12/16/2014 05:47 PM, H. Peter Anvin wrote:
> On 12/16/2014 01:58 AM, Juergen Gross wrote:
>> VMWare seems not to emulate the PAT MSR correctly: reaeding
>> MSR_IA32_CR_PAT returns 0 even after writing another value to it.
>>
>> Detect this bug and don't use the read value if it is 0.
>>
>> Commit bd809af16e3ab1f8d55b3e2928c47c67e2a865d2 ("x86: Enable PAT to
>> use cache mode translation tables") triggers this VMWare bug when the
>> kernel is booted as a VMWare guest.
>>
>> Reported-by: Jongman Heo <jongman.heo@...sung.com>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> Tested-by: Jongman Heo <jongman.heo@...sung.com>
>
> I presume cachability control is irrelevant for a VMware guest?  I'm
> wondering if it would be better to just plain ignore the failure rather
> than disabling PAT.

I'd rather disable PAT as this seems to be like a PAT erratum.

In case you want to keep PAT active I can redo the patch to not rely on
the read MSR value in the non-Xen case (under Xen reading the MSR is the
only way to obtain the correct settings).

Juergen

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