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

[Tomeu Vizoso]

On 6 November 2017 at 23:01, Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxxxxxxxxxx>
>>> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>>
>>
>> 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