linux-kernel - Re: [PATCH 01/11] x86/kernel: remove conditional early remap in parse_e820

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Message-ID: <4D094F1A.3020906@zytor.com>
Date:	Wed, 15 Dec 2010 15:28:26 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:	linux-kernel@...r.kernel.org, sodaville@...utronix.de,
	x86@...nel.org, Dirk Brandewie <dirk.brandewie@...il.com>
Subject: Re: [PATCH 01/11] x86/kernel: remove conditional early remap in parse_e820_ext

On 11/25/2010 09:39 AM, Sebastian Andrzej Siewior wrote:
> parse_setup_data() uses early_memremap() for a PAGE_SIZE mapping in
> order to figure out the type & size. If this mapping is not large enough
> then parse_e820_ext() will remap this area again via early_ioremap()
> since the first mapping is still in use.
> 
> This patch attempts to simplify the handling and parse_e820_ext() does
> not need to worry about the mapping anymore.

I like the fact that this puts all the mapping in the same layer, but I
also think it's unfortunate to discard the optimization of always
mapping the minimum of <header length, rest of page>; your code will
*always* map-unmap-map the code, even in the (presumably very common)
case of the data fitting on a single page.

Furthermore, your code retains a minor bug from the original code, which
is that if the header is not page-aligned, it may be needlessly map more
than one page with unknown content.

The proper way to do it is probably (this is pseudocode):

maplen = max(PAGE_SIZE - (pa_data & ~PAGE_MASK),
             sizeof(struct setup_data));
data = early_memremap(pa_data, maplen);
len = data->len + sizeof(struct setup_data);
if (len > maplen) {
	early_iounmap(pa_data, maplen);
	data = early_memremap(pa_data, maplen);
}

/* ... */

early_iounmap(pa_data, maplen);

I also found your patch description to be needlessly hard to follow.
The key point is that it puts all the map manipulation into
parse_setup_data() where it belongs.  Since you're changing an
interface, however, also do note that you have checked that there are no
other callers to parse_e820_ext().

	-hpa
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