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

[Tom Lendacky]

On 11/6/2017 4:01 PM, Tom Lendacky 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.
> >
> > ÂÂÂÂ-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.

Btw, it was working before because instead of saving off the physical
address as 5997efb96756 now does, it saved off a virtual address that
pointed to physical address 0 (0xffff880000000000) and used that in the
conditional.

Thanks,
Tom


> Thanks,
> Tom
>
> >