linux-kernel - Re: [PATCH] x86/boot: Fix boot failure when SMP MP-table is based at 0

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Message-ID: <ee912340-4b16-2b62-7e92-66dfb4a28ef8@zytor.com>
Date:   Mon, 6 Nov 2017 13:41:09 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tomeu Vizoso <tomeu@...euvizoso.net>
Subject: Re: [PATCH] x86/boot: Fix boot failure when SMP MP-table is based at
 0

On 11/06/17 12:17, Tom Lendacky wrote:
> When crosvm is used to boot a kernel as a VM, the SMP MP-table is found
> at physical address 0x0. This causes mpf_base to be set to 0 and a
> subsequent "if (!mpf_base)" check in default_get_smp_config() results in
> the MP-table not being parsed.  Further into the boot this results in an
> oops when attempting a read_apic_id().
> 
> Add a boolean variable that is set to true when the MP-table is found.
> Use this variable for testing if the MP-table was found so that even a
> value of 0 for mpf_base will result in continued parsing of the MP-table.
> 
> Reported-by: Tomeu Vizoso <tomeu@...euvizoso.net>
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>

Ahem... did anyone ever tell you that this is an epicly bad idea on your
part?  The low megabyte of physical memory has very special meaning on
x86, and deviating from the standard use of this memory is a *very*
dangerous thing to do, and imposing on the kernel a "fake null pointer"
requirement that exists only for the convenience of your particular
brokenness is not okay.

	-hpa