linux-kernel - Re: [GIT PULL] x86/uapi for 3.8

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Message-ID: <50CF5348.2000606@linux.intel.com>
Date:	Mon, 17 Dec 2012 09:15:52 -0800
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...e.hu>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Matt Fleming <matt.fleming@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] x86/uapi for 3.8

On 12/17/2012 09:03 AM, Jan Beulich wrote:
>>>> On 17.12.12 at 17:39, "H. Peter Anvin" <hpa@...ux.intel.com> wrote:
>> Right, I think you nailed this one.  This patch copies PTEs from the
>> kernel PTEs and thus they will have the global bit set.  It obviously
>> makes no sense to *copy* PTEs from the kernel and yet leaving the global
>> bit set, which means there are two ways of fixing it: either sharing
>> page tables and use the cr4.pge off/on trick that Jan mentioned -- this
>> would also be my preference -- and the other is to copy the PTEs but
>> strip the global bit, which has the advantage that the actual kernel
>> mappings will survive.
> 
> PTE copying is only one half of it. I think additionally L4 entries
> get copied for the 1:1 mapping, and you can't strip the global
> bits there without allocating separate page tables.
> 

The point right now is that it *does* allocate separate page tables, but
doesn't take advantage of it.  What I say is I think we should take the
flush for the advantage of sharing page tables.  If we are allocating
new page tables then we should of course make them non-global.

Do we know how often this gets called?  I presume the most common case
is when we have an EFI RTC?  Unless there is a use case where this
happens a lot sharing seems much easier...

	-hpa


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