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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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