linux-kernel - Re: [PATCH v2 17/20] x86: efistub: Check SEV/SNP support while running in the firmware

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Date:   Mon, 22 May 2023 14:48:44 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, Evgeniy Baskov <baskov@...ras.ru>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Mario Limonciello <mario.limonciello@....com>,
        Kees Cook <keescook@...omium.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 17/20] x86: efistub: Check SEV/SNP support while
 running in the firmware

On Fri, May 19, 2023 at 09:04:46AM -0500, Tom Lendacky wrote:
> Deferring the checks is probably the safest thing to do, since that would
> match the way things are done today and known to work. I'm not sure what
> other things might pop up if we stay with this approach, for example, page
> state change calls using the GHCB MSR protocol that also don't save/restore
> the MSR value.
> 
> It is possible to audit these areas and stay with this approach, but I'm
> wondering if that wouldn't be better done as a separate patch series.
> 
> Adding @Joerg for any additional thoughts he might have around this area, too.

If I got it correctly the patch actually moves two things before
ExitBootServices:

	1) SEV features check

	2) SEV initialization

I think it makes a lot of sense to have 1) before ExitBootServices. It
allows to soft-fail in case the kernel does not support all required
SEV-SNP features and move on to a kernel which does. This check also only
needs the SEV_STATUS MSR and not any GHCB calls.

The problem is the GHCB protocol negotiation with the HV, but the GHCB
protocol is downward-compatible, so an older kernel can work with a
newer HV.

But 2) needs to stay after ExitBootServices, as it needs resources owned
by UEFI, e.g. the GHCB MSR and potentially the configured GHCB itself.
Fiddling around with the GHCB MSR while it is still owned by UEFI will
bite us in one or the other way (e.g. UEFI, before ExitBootServices, is
free to take IRQs with handlers that rely on the GHCB MSR content).

Regards,

	Joerg