linux-kernel - Re: [git pull] x86 PAT changes

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Date:	Sat, 26 Apr 2008 16:42:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Gabriel C <nix.or.die@...glemail.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Avi Kivity <avi@...ranet.com>
Subject: Re: [git pull] x86 PAT changes


* Gabriel C <nix.or.die@...glemail.com> wrote:

> Hi Ingo,
> 
> When using an 64bit ( didn't got time to test 32bit now ) kernel[1] 
> with PAT enabled , kvm-intel does not work anymore.
> 
> When modprobing kvm-intel , kvm is saying VT extension is disable by 
> BIOS which isn't true. When disabling PAT again ( no changes to BIOS ) 
> kvm-intel works again here.
> 
> Is that an known problem ?

no, that side-effect was not known. Cc:-ed more folks.

> If you need more infos just let me know.
> 
> Gabriel
> 
> [1]  2.6.25-05096-gb1721d0-dirty
>      with following patches :
>      http://lkml.org/lkml/2008/4/26/37
>      http://lkml.org/lkml/2008/4/26/24
>      http://lkml.org/lkml/2008/4/25/107

thanks - that should be enough for now. We'll try to reproduce these 
problems.

A blind guess: maybe it's the CONFIG_NONPROMISC_DEVMEM somehow breaks 
Qemu. With the patch below you'd be able to disable NONPROMISC_DEVMEM 
without disabling PAT.

	Ingo

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 4d350b5..4aa4180 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1049,9 +1049,9 @@ config MTRR
 	  See <file:Documentation/mtrr.txt> for more information.
 
 config X86_PAT
-	def_bool y
+	bool
 	prompt "x86 PAT support"
-	depends on MTRR && NONPROMISC_DEVMEM
+	depends on MTRR
 	help
 	  Use PAT attributes to setup page level cache control.
 
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