linux-kernel - Re: [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory acceptance

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Message-ID: <20250411191135.GCZ_lpZ-kgr88WNw8e@renoirsky.local>
Date: Fri, 11 Apr 2025 21:11:35 +0200
From: Borislav Petkov <bp@...en8.de>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org,
	x86@...nel.org, mingo@...nel.org, linux-kernel@...r.kernel.org,
	Tom Lendacky <thomas.lendacky@....com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Dionna Amalie Glaze <dionnaglaze@...gle.com>,
	Kevin Loughlin <kevinloughlin@...gle.com>
Subject: Re: [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory
 acceptance

On Fri, Apr 11, 2025 at 09:00:51PM +0200, Ard Biesheuvel wrote:
> ---%<---
> The GHCB shared page method never worked for accepting memory from the
> EFI stub, but this is rarely needed in practice: when using the higher
> level page allocation APIs, the firmware will make sure that memory is
> accepted before it is returned. The only use case for explicit memory
> acceptance by the EFI stub is when populating the 'unaccepted memory'
> bitmap, which tracks unaccepted memory at a 2MB granularity, and so
> chunks of unaccepted memory that are misaligned wrt that are accepted
> without being allocated or used.
> ---%<---
> 
> > I'm somehow missing that aspect of why that change is warranted...
> >
> 
> This never worked correctly for SEV-SNP, we're just lucky the firmware
> appears to accept memory in 2+ MB batches and so these misaligned
> chunks are rare in practice. Tom did manage to trigger it IIUC by
> giving a VM an amount of memory that is not a multiple of 2M.

Can you pls put that lenghty explanation in the commit message?

It is useful and we might go back to it in the future.

Thx.