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

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Date:   Wed, 26 May 2021 10:47:21 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        untaintableangel@...mail.co.uk, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/Kconfig: decrease maximum of X86_RESERVE_LOW to 512K

On Wed, May 26, 2021 at 11:11:00AM +0300, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> After the consolidation of early memory reservations introduced by the
> commit a799c2bd29d1 ("x86/setup: Consolidate early memory reservations")
> the kernel fails to boot if X86_RESERVE_LOW is set to 640K.
> 
> The boot fails because real-time trampoline must be allocated under 1M (or
> essentially under 640K) but with X86_RESERVE_LOW set to 640K the memory is
> already reserved by the time reserve_real_mode() is called.
> 
> Before the reordering of the early memory reservations it was possible to
> allocate from low memory even despite user's request to avoid using that
> memory. This lack of consistency could potentially lead to memory
> corruptions by BIOS in the areas allocated by kernel.

Hmm, so this sounds weird to me: real-time trampoline clearly has
precedence over X86_RESERVE_LOW because you need former to boot the
machine, right?

In that case, real-time trampoline should allocate first and *then* the
rest of low range requested to be reserved should be reserved, no?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette