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

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Date:	Wed, 17 Dec 2014 04:57:02 +0000
From:	Alok Kataria <akataria@...are.com>
To:	"jgross@...e.com" <jgross@...e.com>
CC:	"hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Dunn <ddunn@...are.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"jongman.heo@...sung.com" <jongman.heo@...sung.com>
Subject: Re: [PATCH] x86: don't rely on VMWare emulating PAT MSR correctly

Hi,

On Tue, 2014-12-16 at 10:58 +0100, 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.

Thanks for taking care of this, we are fine with this workaround. On
newer version of our product, where PAT is virtualized correctly on our
platform we will go back to using PAT, I will give it a spin sometime
later to verify it.

Acked-by: Alok N Kataria <akataria@...are.com>

> Reported-by: Jongman Heo <jongman.heo@...sung.com>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> Tested-by: Jongman Heo <jongman.heo@...sung.com>
> ---
>  arch/x86/mm/pat.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index edf299c..7ac6869 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -234,8 +234,13 @@ void pat_init(void)
>  	      PAT(4, WB) | PAT(5, WC) | PAT(6, UC_MINUS) | PAT(7, UC);
>  
>  	/* Boot CPU check */
> -	if (!boot_pat_state)
> +	if (!boot_pat_state) {
>  		rdmsrl(MSR_IA32_CR_PAT, boot_pat_state);
> +		if (!boot_pat_state) {
> +			pat_disable("PAT read returns always zero, disabled.");
> +			return;
> +		}
> +	}
>  
>  	wrmsrl(MSR_IA32_CR_PAT, pat);
>  

Alok