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Date:   Fri, 30 Sep 2022 20:30:33 -0400
From:   Demi Marie Obenour <demi@invisiblethingslab.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Jan Beulich <jbeulich@suse.com>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Kees Cook <keescook@chromium.org>,
        Anton Vorontsov <anton@enomsg.org>,
        Colin Cross <ccross@android.com>,
        Tony Luck <tony.luck@intel.com>,
        Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>
Subject: Re: [PATCH v4 1/2] Avoid using EFI tables Xen may have clobbered
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Date: Fri, 30 Sep 2022 20:30:33 -0400
From: Demi Marie Obenour <demi@invisiblethingslab.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Jan Beulich <jbeulich@suse.com>, xen-devel@lists.xenproject.org,
	linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org,
	Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
	Kees Cook <keescook@chromium.org>,
	Anton Vorontsov <anton@enomsg.org>,
	Colin Cross <ccross@android.com>, Tony Luck <tony.luck@intel.com>,
	Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= <marmarek@invisiblethingslab.com>
Subject: Re: [PATCH v4 1/2] Avoid using EFI tables Xen may have clobbered

On Fri, Sep 30, 2022 at 08:27:09PM +0200, Ard Biesheuvel wrote:
> On Fri, 30 Sept 2022 at 19:12, Demi Marie Obenour wrote:
> > On Fri, Sep 30, 2022 at 06:30:57PM +0200, Ard Biesheuvel wrote:
> > > I know very little about Xen, but based on the context you provided in
> > > this thread, I'd say that the best approach from the Xen side is to
> > > convert all EfiBootServicesData regions that have configuration tables
> > > pointing into them into EfiAcpiReclaimMemory.
> >
> > Should Xen convert the entire region, or should it try to reserve only
> > the memory it needs?  The latter would require it to parse the
> > configuration tables.  Is there a list of configuration tables that can
> > legitimately be in EfiBootServicesData regions?
> >
>=20
> Not really, no. So you would have to convert the entire region
> /unless/ Xen knows the GUID, and therefore knows how to derive the
> size of the table, allowing it to reserve memory more conservatively.
> However, I doubt whether this is worth it: splitting entries implies
> rewriting the memory map, which is a thing I'd rather avoid if I were
> in your shoes.

I actually wonder if Xen needs to reserve *all* of EfiBootServicesData.
The reason is that some (probably buggy) firmware may store ACPI tables
there, and Xen does not have an ACPI implementation.  From my
perspective, a much safer approach would be to pass all of
EfiBootServicesData memory directly to dom0, and have dom0 give Xen back
what it doesn=E2=80=99t wind up using.  That allows dom0=E2=80=99s memory r=
eservation
code to work properly, which it currently does not.
--=20
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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