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

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Message-ID: <CAAObsKDPx2NW3prEQyF5ojzyM4J=5L+qzzm9uX=7T2XvWeF9dQ@mail.gmail.com>
Date:   Wed, 8 Nov 2017 09:37:06 +0100
From:   Tomeu Vizoso <tomeu@...euvizoso.net>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/boot: Fix boot failure when SMP MP-table is based at 0

On 6 November 2017 at 23:01, Tom Lendacky <thomas.lendacky@....com> wrote:
> On 11/6/2017 3:41 PM, H. Peter Anvin wrote:
>>
>> 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
>
>
> That was my initial thought... what was something doing down at the start
> of memory.  But when I looked at default_find_smp_config() it specifically
> scans the bottom 1K for a an MP-table signature. I was hoping to get some
> feedback as to whether this would really be an acceptable thing to do. So
> I'm good with this patch being rejected, but the change I made in
>
> 5997efb96756 ("x86/boot: Use memremap() to map the MPF and MPC data")
>
> does break something that was working before.

Do I understand correctly that the best we can do right now is
reverting 5997efb96756?

Thanks,

Tomeu