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

[Tom Lendacky <thomas.lendacky@amd.com>]

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@tomeuvizoso.net>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.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.

Thanks,
Tom

>