Re: BIOS overwritten during resume (was: Re: Asus L5D resume on battery power)

["Rafael J. Wysocki" <rjw@sisk.pl>]

Hi,

On Thursday, 3 of March 2005 00:54, Pavel Machek wrote:
> Hi!
> 
> > > > It seems that we write to the BIOS while moving the image, at least on my box,
> > > > which is quite not correct, IMO.
> > [-- snip --]
> > > > 
> > > > IMO this may lead to unexpected results, like the mysterious reboots during
> > > > resume.
> > > 
> > > Well, I always thought that ROM-BIOS is expected to
> > > be... well... read-only? Can you really write to your BIOS? [I know
> > > about Flash-BIOSen, but they are certainly not writable by "normal"
> > > write.] Plus we should overwrite it with same values...
> > > 
> > > Anyway, IMO bios should be marked as reserved (and we should not be
> > > touching reserved pages). Can you verify that your BIOS is properly
> > > marked reserved? [Ccing l-k, this might be interesting.]
> > 
> > It is, but that's even more interesting.  Namely, the BIOS-e820 memory map
> > looks as follows:
> > 
> > BIOS-provided physical RAM map:
> >  BIOS-e820: 0000000000000000 - 000000000009fc00 (usable)
> >  BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
> >  BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved)
> >  BIOS-e820: 0000000000100000 - 000000001ff40000 (usable)
> >  BIOS-e820: 000000001ff40000 - 000000001ff50000 (ACPI data)
> >  BIOS-e820: 000000001ff50000 - 0000000020000000 (ACPI NVS)
> >  BIOS-e820: 00000000fec00000 - 00000000fec01000 (reserved)
> >  BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
> > 
> > and the pages in the first two reserved areas are properly marked as reserved.
> > Moreover, there is an apparent hole between a0000 and e0000 which also
> > is marked as reserved.  However, we treat all of these pages as saveable
> > (a page is treated as non-saveable if it is marked as reserved _and_ satisfies
> > specific condition which is met by only one page on my system).  Consequently,
> > during resume we try to overwrite all of these pages and we not only try to
> > write to (theoretically) read-only areas, but also we try to write to where there's
> > nothing. :-)
> > 
> > Still, there are almost 3500 reserved pages on my system and I'm not sure that
> > we don't need to save at least some of them.  Anyway, I'm attaching the full
> > list of pages that are marked as reserved and treated as saveable, so they get
> > overwritten during resume.  Please have a look (these are virtual x86-64
> > addresses, but the mapping is straightforward).
> 
> [looking at code]
> 
> IIRC kernel code/data is marked as PageReserved(), that's why we need
> to save that :(. Not sure what to do with data e820 marked as
> reserved...

Perhaps we need another page flag, like PG_readonly, and mark the pages
reserved by the e820 as PG_reserved | PG_readonly (the same for the areas
that are not returned by e820 at all).  Would that be acceptable?

Greets,
Rafael


-- 
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- That depends a good deal on where you want to get to.
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