Re: [PATCH v3 1/2] schemas: Add a schema for memory map

[Simon Glass]

(Adding a few more, will respond soon)

- Simon

On Thu, 24 Aug 2023 at 03:10, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Wed, 23 Aug 2023 at 22:04, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > On Wed, 23 Aug 2023 at 08:24, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > >
> > > On Wed, 23 Aug 2023 at 10:59, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > >
> > > > On Tue, Aug 22, 2023 at 02:34:42PM -0600, Simon Glass wrote:
> > > > > The Devicetree specification skips over handling of a logical view of
> > > > > the memory map, pointing users to the UEFI specification.
> > > > >
> > > > > It is common to split firmware into 'Platform Init', which does the
> > > > > initial hardware setup and a "Payload" which selects the OS to be booted.
> > > > > Thus an handover interface is required between these two pieces.
> > > > >
> > > > > Where UEFI boot-time services are not available, but UEFI firmware is
> > > > > present on either side of this interface, information about memory usage
> > > > > and attributes must be presented to the "Payload" in some form.
> > >
> > > Not quite.
> > >
> > > This seems to be intended for consumption by Linux booting in ACPI
> > > mode, but not via UEFI, right?
> >
> > Actually, this is for consumption by firmware. The goal is to allow
> > edk2 to boot into U-Boot and vice versa, i.e. provide some
> > interoperability between firmware projects. I will use the "Platform
> > Init" and "Payload" terminology here too.
> >
>
> OK. It was the cc to linux-acpi@ and the authors of the
> ACPI/SMBIOS-without-UEFI patches that threw me off here.
>
> If we are talking about an internal interface for firmware components,
> I'd be inclined to treat this as an implementation detail, as long as
> the OS is not expected to consume these DT nodes.
>
> However, I struggle to see the point of framing this information as a
> 'UEFI memory map'. Neither EDK2 nor u-boot consume this information
> natively, and there is already prior art in both projects to consume
> nodes following the existing bindings for device_type=memory and the
> /reserved-memory node. UEFI runtime memory is generally useless
> without UEFI runtime services, and UEFI boot services memory is just
> free memory.
>
> There is also an overlap with the handover between secure and
> non-secure firmware on arm64, which is also DT based, and communicates
> available memory as well as RAM regions that are reserved for firmware
> use.
>
> In summary, I don't see why a non-UEFI payload would care about UEFI
> semantics for pre-existing memory reservations, or vice versa. Note
> that EDK2 will manage its own memory map, and expose it via UEFI boot
> services and not via DT.
>
> ...
> >
> > There is no intent to implement the UEFI spec, here. It is simply that
> > some payloads (EDK2) are used to having this information.
> >
> > Imagine splitting EDK2 into two parts, one of which does platform init
> > and the other which (the payload) boots the OS. The payload wants
> > information from Platform Init and it needs to be in a devicetree,
> > since that is what we have chosen for this interface. So to some
> > extent this is unrelated to whether you have EFI boot services. We
> > just need to be able to pass the information across the interface.
> > Note that the user can (without recompilation, etc.) replace the
> > second part with U-Boot (for example) and it must still work.
> >
>
> OK, so device tree makes sense for this. This is how I implemented the
> EDK2 port that targets QEMU/mach-virt - it consumes the DT to discover
> the UART, RTC,, memory, PCI host bridge, etc.
>
> But I don't see a use case for a UEFI memory map here.
>
>
> > >
> > > >
> > > > Today Linux does that by passing:
> > > >
> > > >   /chosen/linux,uefi-mmap-start
> > > >   /chosen/linux,uefi-mmap-size
> > > >   /chosen/linux,uefi-mmap-desc-size
> > > >   /chosen/linux,uefi-mmap-desc-ver
> > > >
> > > > ... or /chosen/xen,* variants of those.
> > > >
> > > > Can't we document / genericise that?
> >
> > That seems to me to be the fields from the EFI memory-map call, but
> > where is the actual content? I looked in the kernel but it seems to be
> > an internal interface (between the stub and the kernel)?
> >
> > > >
> > >
> > > Given the ACPI angle, promoting this to external ABI would introduce a
> > > DT dependency to ACPI boot. So we'll at least have to be very clear
> > > about which takes precedence, or maybe disregard everything except the
> > > /chosen node when doing ACPI boot?
> > >
> > > This also argues for not creating an ordinary binding for this (i.e.,
> > > describing it as part of the platform topology), but putting it under
> > > /chosen as a Linux-only boot tweak.
> > >
> > > > Pointing to that rather than re-encoding it in DT means that it stays in-sync
> > > > with the EFI spec and we won't back ourselves into a corner where we cannot
> > > > encode something due to a structural difference. I don't think it's a good idea
> > > > to try to re-encode it, or we're just setting ourselves up for futher pain.
> > > >
> > >
> > > What I would prefer is to formalize pseudo-EFI boot and define the
> > > bare required minimum (system table + memory map + config tables) in
> > > an arch-agnostic manner. That way, the only thing that needs to be
> > > passed via DT/boot_params/etc is the (pseudo-)EFI system table
> > > address, and everything else (SMBIOS, ACPI as well as the EFI memory
> > > map and even the initrd) can be passed via config tables as usual, all
> > > of which is already supported in (mostly) generic kernel code.
> > >
>
> <snip some lines>
>
> >
> > Here I believe you are talking about booting the kernel in EFI mode,
> > but that is not the intent of this patch. This is all about things
> > happening in firmware. Now, if the payload (second) part of the
> > firmware decides it wants to offer EFI boot services and boot the
> > kernel via the EFI stub, then it may very well pack this information
> > (with a few changes) into a system table and make it available to the
> > kernel stub. But by then this FDT binding is irrelevant, since it has
> > served its purpose (which, to reiterate, is to facilitate information
> > passage from platform init to 'payload').
> >
>
> Indeed. As long as this binding is never consumed by the OS, I don't
> have any objections to it - I just fail to see the point.