linux-kernel - Re: [PATCH v2 3/3] x86/boot: Implement early memory acceptance for SEV-SNP

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Message-ID: <CAMj1kXHcdT8X8uUyd+9eEF1Nc=CBNj2M4z172DUU7g_pJ5BHuw@mail.gmail.com>
Date: Mon, 7 Apr 2025 19:45:59 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Ingo Molnar <mingo@...nel.org>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Dionna Amalie Glaze <dionnaglaze@...gle.com>, Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, x86@...nel.org, 
	Tom Lendacky <thomas.lendacky@....com>, Borislav Petkov <bp@...en8.de>, 
	Kevin Loughlin <kevinloughlin@...gle.com>
Subject: Re: [PATCH v2 3/3] x86/boot: Implement early memory acceptance for SEV-SNP

On Mon, 7 Apr 2025 at 19:33, Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> On Mon, Apr 07, 2025 at 07:21:17PM +0200, Ard Biesheuvel wrote:
> > On Mon, 7 Apr 2025 at 18:44, Ingo Molnar <mingo@...nel.org> wrote:
> > >
> > >
> > > * Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:
> > >
> > > > On Fri, Apr 04, 2025 at 08:07:03AM -0700, Dionna Amalie Glaze wrote:
> > > > > If the GHCB is available, we should always prefer it.
> > > >
> > > > I believe we should consider the cost of code duplication in this
> > > > situation.
> > > >
> > > > If the non-early version is only used in the kexec path, it will not be
> > > > tested as frequently and could be more easily broken. I think it would be
> > > > acceptable for kexec to be slightly slower if it results in more
> > > > maintainable code.
> > >
> > > Absolutely so.
> > >
> >
> > It would be nice if someone could quantify 'slightly slower' - I am
> > leaning to the same conclusion but I have no clue what the actual
> > performance impact is.
>
> If we can survive the performance of the initial boot, we can live with it
> for kexec.
>

The initial boot does not occur via the decompressor, but via the EFI
stub, where memory acceptance is handled by the firmware (as it
should).

Given that the traditional decompressor carves out an allocation from
the raw E820 map without using any of the higher level APIs, it has to
accept the memory itself if it is marked as unaccepted in the table.

Perhaps the decompressor should try to avoid unaccepted memory?