Re: [PATCH] x86/sev: Apply RMP table fixups for kexec.

"Kalra, Ashish" <ashish.kalra@xxxxxxx> · Tue, 2 Apr 2024 12:11:03 -0500

On 4/2/2024 12:06 PM, Kalra, Ashish wrote:

On 4/2/2024 11:34 AM, bp@xxxxxxxxxx wrote:

From: Borislav Petkov <bp@xxxxxxxxx>

On Tue, Apr 02, 2024 at 10:54:50AM -0500, Tom Lendacky wrote:

There's no requirement from a hardware/RMP usage perspective that 

requires a

2MB alignment, so BIOS is not doing anything wrong. The problem occurs

because kexec is initially using 2MB mappings that overlap the start 

and/or

end of the RMP which then results in an RMP fault when memory within 

one of

those 2MB mappings, that is not part of the RMP, is referenced.

Then this explanation is misleading. And that whole bla about alignment
is nonsense either.

Additionally, we have BIOSes out there since Milan that don't do 

this 2MB

alignment. And do you really trust that BIOS will do this properly 

all the

time?

I don't trust the BIOS to do anything properly.

So why isn't the fix for this simply to reserve the space for the RMP
table to start at 2M page - even if it doesn't - and to cover the last
chunk *also* with a 2M page and be done with it?

But, it is the BIOS which reserves the space for the RMP table.

And if you mean the reservation in the kernel page tables (directmap) 

then that will not help as kexec uses it's own identity mapped page 

tables.

Not this silly overriding dance.

This overriding dance is required to fixup all the three kexec tables, 

as there is no interface/API available to do modifications/fixups in 

all the three possible kexec tables.

I meant the three e820 tables out of which two are used for kexec, the 

e820_table_firmware and e820_table_kexec.

Thanks, Ashish

Thx.