linux-kernel - Re: [RFC PATCH 0/7] Improve swiotlb performance by using physical addresses

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Message-ID: <50707ECD.2070300@zytor.com>
Date:	Sat, 06 Oct 2012 11:56:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Alexander Duyck <alexander.h.duyck@...el.com>,
	konrad.wilk@...cle.com, tglx@...utronix.de, mingo@...hat.com,
	rob@...dley.net, akpm@...ux-foundation.org, joerg.roedel@....com,
	bhelgaas@...gle.com, shuahkhan@...il.com,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	x86@...nel.org, torvalds@...ux-foundation.org
Subject: Re: [RFC PATCH 0/7] Improve swiotlb performance by using physical
 addresses

On 10/06/2012 10:57 AM, Andi Kleen wrote:
> 
> Maybe it's just me, but that's somehow sad for one if() and a su
> btraction
> 
> BTW __pa used to be a simple subtraction, the if () was just added to
> handle the few call sites for x86-64 that do __pa(&text_symbol).
> Maybe we should just go back to the old __pa_symbol() for those cases,
> then __pa could be the simple subtraction it used to was again
> and it could be inlined and everyone would be happy.
> 

I wonder how much the double-mapping actually buys us, if we could get a
PIE-type memory model that doesn't try to create GOT references.

We would unambiguously lose out the cases where we currently can do
symbol(index) but I wonder how much the impact actually is.

The advantage would be that it would give us way more room to do kernel
space ASR.

	-hpa


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