linux-kernel - Re: [PATCH] x86/pci: make pci_mem

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Message-ID: <49E4FFBC.4040901@zytor.com>
Date:	Tue, 14 Apr 2009 14:27:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Yinghai Lu <yinghai@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-pci@...r.kernel.org, yannick.roehlly@...e.fr
Subject: Re: [PATCH] x86/pci: make pci_mem_start to be aligned only

Linus Torvalds wrote:
> The reason? We've definitely seen ACPI code or integrated graphics stuff 
> that steals a lot of memory at the end, which means that end-of-RAM might 
> be not at 2GB, but at 2GB-16MB-1MB, for example (1MB of "ACPI data", and 
> 16MB of "stolen video ram").

This is pretty much standard these days.  It's hard to implement ACPI 
without doing so.  Throw in the SMI T-seg for even more fun.

> Now, the BIOS _hopefully_ marks those areas clearly reserved, and as a 
> result we don't end up allocating PCI data in there, but the gap was there 
> literally to make sure we always leave that gap, very much on purpose.

It would be nice if we would mark that memory reserved ourselves.

> thing, so that if the gap is large, then we'll certainly get to 32MB too, 
> but I think your patch matters the most exactly when the gap is small. 
> Maybe we could just raise the initial minimum rounding from 1MB to 32MB?

Since we're talking about address space, not actual memory, it seems 
rather hard to end up in a situation where either one of these is not true:

- we will have real hardware demand for a large alignment datum.
- we will have so much address space available that it doesn't matter.

The latter case would be e.g. a machine with a today-anemic handful of 
megabytes of RAM.

	-hpa[1]

[1] who remembers running a Linux server on a 0.59 bogomips i386/16 with
     3 MB of half-speed memory...

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