linux-kernel - RE: [PATCH] x86/Kconfig: decrease maximum of X86_RESERVE

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Message-ID: <8f74d4bd0d97445c8a976eab44fc9372@AcuMS.aculab.com>
Date:   Mon, 31 May 2021 09:32:22 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'H. Peter Anvin'" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        "Mike Rapoport" <rppt@...nel.org>
CC:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        "untaintableangel@...mail.co.uk" <untaintableangel@...mail.co.uk>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] x86/Kconfig: decrease maximum of X86_RESERVE_LOW to 512K

From: H. Peter Anvin
> Sent: 28 May 2021 03:13
....
> BIOSes have been known to clobber more than 64K. They aren't supposed to
> clobber any.

They probably shouldn't need anything above the base of the DOS
transient program area preserved.
Can't remember where that is though :-(

It is hard enough finding a safe memory area for the MBR
code to relocate itself to before loading the PBR.
Both the MBR and PBR load at the same address - 0xc00.

> 640K is the limit because that is the address of the EGA/VGA frame
> buffer. In the words of Bill Gates "640K ought to be enough for anyone."

I thought the original memory map allocated 512K for memory
and 512k for memory mapped I/O.
No one could afford more then 512K DRAM :-)

The 640K limit appears because nothing was actually mapped
as the bottom of the 'I/O area' so memory could expand up
that far.

	David

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