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

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Date: Tue,  2 Apr 2024 18:34:12 +0200
From: bp@...nel.org
To: thomas.lendacky@....com
Cc: ashish.kalra@....com,
	bp@...en8.de,
	bp@...nel.org,
	linux-kernel@...r.kernel.org,
	linux-tip-commits@...r.kernel.org,
	michael.roth@....com,
	x86@...nel.org
Subject: Re: [PATCH] x86/sev: Apply RMP table fixups for kexec.

From: Borislav Petkov <bp@...en8.de>

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?

Not this silly overriding dance.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette