linux-kernel - Re: [PATCH v2] x86/efi: Safely enable unaccepted memory in UEFI

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Message-ID: <CAMj1kXG=rec5HSxfteQnc6ZZ0RnQZq+-wX6HNG5gm4tiSOuH_g@mail.gmail.com>
Date:   Wed, 18 Jan 2023 16:46:09 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Mark Rutland <mark.rutland@....com>,
        Borislav Petkov <bp@...en8.de>,
        Gerd Hoffmann <kraxel@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        x86@...nel.org, jiewen.yao@...el.com, devel@...2.groups.io,
        "Min M. Xu" <min.m.xu@...el.org>,
        James Bottomley <jejb@...ux.ibm.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2] x86/efi: Safely enable unaccepted memory in UEFI

On Wed, 18 Jan 2023 at 16:41, Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 1/18/23 07:09, Ard Biesheuvel wrote:
> > However, I guess we're at a point where SEV and TDX really want
> > different solutions, so I think divergence might be the way to
> > proceed.
>
> I don't think they want different things really.
>
> TDX doesn't need this protocol.  It sounds like SEV does need it,
> though.  That doesn't mean they really diverge.  They're *both* going to
> have to poke at this protocol knob to get the firmware to not accept the
> memory.
>

No, on TDX, the firmware would never accept all memory. On SEV, it
would only do so if the protocol has not been called prior to the call
to ExitBootServices().

> This does slightly change the motivation for doing explicit unaccepted
> memory support in the kernel.
>

Not on TDX.

> I also don't know _quite_ how this will look to a guest.  For instance,
> will they see different memory maps based on which protocol they are
> using?  I assume so, but didn't see any of that explicitly mentioned in
> this patch.

The EFI memory map will not contain ranges of type
EFI_UNACCEPTED_MEMORY if the memory was accepted on behalf of the OS
by the firmware. That is the point, really, as non-enlightened OSes
will ignore those.