[TF-A] Proposal: TF-A to adopt hand-off blocks (HOBs) for information passing between boot stages

Manish Pandey2 Manish.Pandey2 at arm.com
Thu Apr 8 13:55:45 CEST 2021


From TF-A project point of view, we prefer to use existing mechanism to pass parameters across boot stages using linked list of tagged elements (as suggested by Julius). It has support for both generic and SiP-specific tags. Having said that, it does not stop partners to introduce new mechanisms suitable for their usecase in platform port initially and later move to generic code if its suitable for other platforms.

To start with, Ampere can introduce a platform specific implementation of memory tag(speed/NUMA topology etc) which can be evaluated and discussed for generalization in future. The tag will be populated in BL2 stage and can be forwarded to further stages(and to BL33) by passing the head of list pointer in one of the registers. Initially any register can be used but going forward a standardization will be needed.

The U-boot bloblist mentioned by Simon is conceptually similar to what TF-A is using,  if there is consensus of using bloblist/taglist then TF-A tag list may be enhanced to take best of both the implementations.

One of the potential problems of having structure used in different projects is maintainability, this can be avoided by having a single copy of these structures in TF-A (kept inside "include/export" which intended to be used by other projects.)

Regarding usage of either UUID or tag, I echo the sentiments of Simon and Julius to keep it simple and use tag values.

Looking forward to having further discussions on zoom call today.

Manish P

From: TF-A <tf-a-bounces at lists.trustedfirmware.org> on behalf of Julius Werner via TF-A <tf-a at lists.trustedfirmware.org>
Sent: 25 March 2021 02:43
To: Simon Glass <sjg at chromium.org>
Cc: Harb Abdulhamid OS <abdulhamid at os.amperecomputing.com>; Boot Architecture Mailman List <boot-architecture at lists.linaro.org>; tf-a at lists.trustedfirmware.org <tf-a at lists.trustedfirmware.org>; U-Boot Mailing List <u-boot at lists.denx.de>; Paul Isaac's <paul.isaacs at linaro.org>; Ron Minnich <rminnich at google.com>
Subject: Re: [TF-A] Proposal: TF-A to adopt hand-off blocks (HOBs) for information passing between boot stages

Just want to point out that TF-A currently already supports a (very simple) mechanism like this:


It's just a linked list of tagged elements. The tag space is split into TF-A-wide generic tags and SiP-specific tags (with plenty of room to spare if more areas need to be defined -- a 64-bit tag can fit a lot). This is currently being used by some platforms that run coreboot in place of BL1/BL2, to pass information from coreboot (BL2) to BL31.

I would echo Simon's sentiment of keeping this as simple as possible and avoiding complicated and bloated data structures with UUIDs. You usually want to parse something like this as early as possible in the passed-to firmware stage, particularly if the structure encodes information about the debug console (like it does for the platforms I mentioned above). For example, in BL31 this basically means doing it right after moving from assembly to C in bl31_early_platform_setup2() to get the console up before running anything else. At that point in the BL31 initialization, the MMU and caches are disabled, so data accesses are pretty expensive and you don't want to spend a lot of parsing effort or calculate complicated checksums or the like. You just want something extremely simple where you ideally have to touch every data word only once.

On Wed, Mar 24, 2021 at 5:06 PM Simon Glass via TF-A <tf-a at lists.trustedfirmware.org<mailto:tf-a at lists.trustedfirmware.org>> wrote:
Hi Harb,

On Wed, 24 Mar 2021 at 11:39, Harb Abdulhamid OS <abdulhamid at os.amperecomputing.com<mailto:abdulhamid at os.amperecomputing.com>> wrote:

Hello Folks,

Appreciate the feedback and replies on this.  Glad to see that there is interest in this topic. 😊

I try to address the comments/feedback from Francois and Simon below….

@François Ozog<mailto:francois.ozog at linaro.org> – happy to discuss this on a zoom call.  I will make that time slot work, and will be available to attend April 8, 4pm CT.

Note that I’m using the term “HOB” here more generically, as there are typically vendor specific structures beyond the resource descriptor HOB, which provides only a small subset of the information that needs to be passed between the boot phases.

The whole point here is to provide mechanism to develop firmware that we can build ARM Server SoC’s that support *any* BL33 payload (e.g. EDK2, AptioV, CoreBoot, and maybe even directly boot strapping LinuxBoot at some point).   In other-words, we are trying to come up with a TF-A that would be completely agnostic to the implementation of BL33 (i.e. BL33 is built completely independently by a separate entity – e.g. an ODM/OEM).

Keep in mind, in the server/datacenter market segment we are not building vertically integrated systems with a single entity compiling firmware/software stacks like most folks in TF-A have become use to.  There are two categories of higher level firmware code blobs in the server/datacenter model:

  1.  “SoC” or “silicon” firmware – in TF-A this may map to BL1, BL2, BL31, and *possibly* one or more BL32 instances
  2.  “Platform” or “board” firmware – in TF-A this may map to BL33 and *possibly* one or more BL32 instances.

Even the platform firmware stack could be further fragmented by having multiple entities involved in delivering the entire firmware stack: IBVs, ODMs, OEMs, CSPs, and possibly even device vendor code.

To support a broad range of platform designs with a broad range of memory devices, we need a crisp and clear contract between the SoC firmware that initializes memory (e.g. BL2) and how that platform boot firmware (e.g. BL33) gathers information about what memory that was initialized, at what speeds, NUMA topology, and many other relevant information that needs to be known and comprehended by the platform firmware and eventually by the platform software.

I understand the versatility of DT, but I see two major problems with DT:

  *   DT requires more complicated parsing to get properties, and even more complex to dynamically set properties – this HOB structures may need to be generated in boot phases where DDR is not available, and therefore we will be extremely memory constrained.
  *   DT is probably overkill for this purpose – We really just want a list of pointers to simple C structures that code cast (e.g. JEDEC SPD data blob)

I think that we should not mix the efforts around DT/ACPI specs with what we are doing here, because those specs and concepts were developed for a completely different purpose (i.e. abstractions needed for OS / RTOS software, and not necessarily suitable for firmware-to-firmware hand-offs).

Frankly, I would personally push back pretty hard on defining SMC’s for something that should be one way information passing.  Every SMC we add is another attack vector to the secure world and an increased burden on the folks that have to do security auditing and threat analysis.  I see no benefit in exposing these boot/HOB/BOB structures at run-time via SMC calls.

Please do let me know if you disagree and why.  Look forward to discussing on this thread or on the call.

@Simon Glass<mailto:sjg at chromium.org>   - Thanks for the pointer to bloblist.   I briefly reviewed and it seems like a good baseline for what we may be looking for.

That being said, I would say that there is some benefit in having some kind of unique identifiers (e.g. UUID or some unique signature) so that we can tie standardized data structures (based on some future TBD specs) to a particular ID.  For example, if the TPM driver in BL33 is looking for the TPM structure in the HOB/BOB list, and may not care about the other data blobs.  The driver needs a way to identify and locate the blob it cares about.

The tag is intended to serve that purpose, although perhaps it should switch from an auto-allocating enum to one with explicit values for each entry and a range for 'local' use.

I guess we can achieve this with the tag, but the problem with tag when you have eco-system with a lot of parties doing parallel development, you can end up with tag collisions and folks fighting about who has rights to what tag values.  We would need some official process for folks to register tags for whatever new structures we define, or maybe some tag range for vendor specific structures.  This comes with a lot of pain and bureaucracy.  On the other hand, UUID has been a proven way to make it easy to just define your own blobs with *either* standard or vendor specific structures without worry of ID collisions between vendors.

True. I think the pain is overstated, though. In this case I think we actually want something that can be shared between projects and orgs, so some amount of coordination could be considered a benefit. It could just be a github pull request. I find the UUID unfriendly and not just to code size and eyesight! Trying to discover what GUIDs mean or are valid is quite tricky. E.g. see this code:

EFI_GUID(0xd038747c, 0xd00c, 0x4980, \
0xb3, 0x19, 0x49, 0x01, 0x99, 0xa4, 0x7d, 0x55)

static struct guid_name {
   efi_guid_t guid;
   const char *name;
} guid_name[] = {
   { FSP_GRAPHICS_INFO_HOB_GUID, "Graphics info" },
(never figured out what those two are)

   { {}, "zero-guid" },

static const char *guid_to_name(const efi_guid_t *guid)
   struct guid_name *entry;

   for (entry = guid_name; entry->name; entry++) {
      if (!guidcmp(guid, &entry->guid))
         return entry->name;

   return NULL;

Believe it or not it took a fair bit of effort to find just that small list, with nearly every one in a separate doc, from memory.

We can probably debate whether there is any value in GUID/UUID or not during the call… but again, boblist seems like a reasonable starting point as an alternative to HOB.

Indeed. There is certainly value in both approaches.




From: François Ozog <francois.ozog at linaro.org<mailto:francois.ozog at linaro.org>>
Sent: Tuesday, March 23, 2021 10:00 AM
To: François Ozog <francois.ozog at linaro.org<mailto:francois.ozog at linaro.org>>; Ron Minnich <rminnich at google.com<mailto:rminnich at google.com>>; Paul Isaac's <paul.isaacs at linaro.org<mailto:paul.isaacs at linaro.org>>
Cc: Simon Glass <sjg at chromium.org<mailto:sjg at chromium.org>>; Harb Abdulhamid OS <abdulhamid at os.amperecomputing.com<mailto:abdulhamid at os.amperecomputing.com>>; Boot Architecture Mailman List <boot-architecture at lists.linaro.org<mailto:boot-architecture at lists.linaro.org>>; tf-a at lists.trustedfirmware.org<mailto:tf-a at lists.trustedfirmware.org>
Subject: Re: [TF-A] Proposal: TF-A to adopt hand-off blocks (HOBs) for information passing between boot stages

+Ron Minnich<mailto:rminnich at google.com> +Paul Isaac's<mailto:paul.isaacs at linaro.org>

Adding Ron and Paul because I think this interface should be also benefiting LinuxBoot efforts.

On Tue, 23 Mar 2021 at 11:17, François Ozog via TF-A <tf-a at lists.trustedfirmware.org<mailto:tf-a at lists.trustedfirmware.org>> wrote:


I propose we cover the topic at the next Trusted Substrate<https://collaborate.linaro.org/display/TS/Trusted+Substrate+Home>    zoom call<https://linaro-org.zoom.us/j/94563644892> on April 8th 4pm CET.

The agenda:

ABI between non-secure firmware and the rest of firmware (EL3, S-EL1, S-EL2, SCP) to adapt hardware description to some runtime conditions.

runtime conditions here relates to DRAM size and topology detection, secure DRAM memory carvings, PSCI and SCMI interface publishing.

For additional background on existing metadata: UEFI Platform Initialization Specification Version 1.7<https://uefi.org/sites/default/files/resources/PI_Spec_1_7_final_Jan_2019.pdf>, 5.5 Resource Descriptor HOB

Out of the ResourceType we care about is EFI_RESOURCE_SYSTEM_MEMORY.

This HOB lacks memory NUMA attachment or something that could be related to fill SRAT table for ACPI or relevant DT proximity domains.

HOB is not consistent accros platforms: some platforms (Arm) lists memory from the booting NUMA node, other platforms (x86) lists all memory from all NUMA nodes. (At least this is the case on the two platforms I tested).

There are two proposals to use memory structures from SPL/BLx up to the handover function (as defined in the Device Tree technical report<https://docs.google.com/document/d/1CLkhLRaz_zcCq44DLGmPZQFPbYHOC6nzPowaL0XmRk0/edit?usp=sharing>) which can be U-boot (BL33 or just U-Boot in case of SPL/U-Boot scheme) or EDK2.

I would propose we also discuss possibility of FF-A interface to actually query information or request actions to be done (this is a model actually used in some SoCs with proprietary SMC calls).

Requirements (to be validated):

- ACPI and DT hardware descriptions.

- agnostic to boot framework (SPL/U-Boot, TF-A/U-Boot, TF-A/EDK2)

- agnostic to boot framework (SPL/U-Boot, TF-A/U-Boot, TF-A/EDK2, TF-A/LinuxBoot)

- at least allows complete DRAM description and "persistent" usage (reserved areas for secure world or other usages)

- support secure world device assignment



On Mon, 22 Mar 2021 at 19:56, Simon Glass <sjg at chromium.org<mailto:sjg at chromium.org>> wrote:


Can I suggest using bloblist for this instead? It is lightweight,
easier to parse, doesn't have GUIDs and is already used within U-Boot
for passing info between SPL/U-Boot, etc.

Docs here: https://github.com/u-boot/u-boot/blob/master/doc/README.bloblist
Header file describes the format:

Full set of unit tests:


On Mon, 22 Mar 2021 at 23:58, François Ozog <francois.ozog at linaro.org<mailto:francois.ozog at linaro.org>> wrote:
> +Boot Architecture Mailman List <boot-architecture at lists.linaro.org<mailto:boot-architecture at lists.linaro.org>>
> standardization is very much welcomed here and need to accommodate a very
> diverse set of situations.
> For example, TEE OS may need to pass memory reservations to BL33 or
> "capture" a device for the secure world.
> I have observed a number of architectures:
> 1) pass information from BLx to BLy in the form of a specific object
> 2) BLx called by BLy by a platform specific SMC to get information
> 3) BLx called by BLy by a platform specific SMC to perform Device Tree
> fixups
> I also imagined a standardized "broadcast" FF-A call so that any firmware
> element can either provide information or "do something".
> My understanding of your proposal is about standardizing on architecture 1)
> with the HOB format.
> The advantage of the HOB is simplicity but it may be difficult to implement
> schemes such as pruning a DT because device assignment in the secure world.
> In any case, it looks feasible to have TF-A and OP-TEE complement the list
> of HOBs to pass information downstream (the bootflow).
> It would be good to start with building the comprehensive list of
> information that need to be conveyed between firmware elements:
> information.    | authoritative entity | reporting entity | information
> exchanged:
> dram               | TFA                       | TFA                   |
> <format to be detailed, NUMA topology to build the SRAT table or DT
> equivalent?>
> PSCI               | SCP                      | TFA?                 |
> SCMI              | SCP or TEE-OS    | TFA? TEE-OS?|
> secure SRAM | TFA.                      | TFA.                  |
> secure DRAM | TFA? TEE-OS?    | TFA? TEE-OS? |
> other?             |                               |
>    |
> Cheers
> FF
> On Mon, 22 Mar 2021 at 09:34, Harb Abdulhamid OS via TF-A <
> tf-a at lists.trustedfirmware.org<mailto:tf-a at lists.trustedfirmware.org>> wrote:
> > Hello Folks,
> >
> >
> >
> > I'm emailing to start an open discussion about the adoption of a concept
> > known as "hand-off blocks" or HOB to become a part of the TF-A Firmware
> > Framework Architecture (FFA).  This is something that is a pretty major
> > pain point when it comes to the adoption of TF-A in ARM Server SoC’s
> > designed to enable a broad range of highly configurable datacenter
> > platforms.
> >
> >
> >
> >
> >
> > What is a HOB (Background)?
> >
> > ---------------------------
> >
> > UEFI PI spec describes a particular definition for how HOB may be used for
> > transitioning between the PEI and DXE boot phases, which is a good
> > reference point for this discussion, but not necessarily the exact solution
> > appropriate for TF-A.
> >
> >
> >
> > A HOB is simply a dynamically generated data structure passed in between
> > two boot phases.  This is information that was obtained through discovery
> > and needs to be passed forward to the next boot phase *once*, with no API
> > needed to call back (e.g. no call back into previous firmware phase is
> > needed to fetch this information at run-time - it is simply passed one time
> > during boot).
> >
> >
> >
> > There may be one or more HOBs passed in between boot phases.  If there are
> > more than one HOB that needs to be passed, this can be in a form of a "HOB
> > table", which (for example) could be a UUID indexed array of pointers to
> > HOB structures, used to locate a HOB of interest (based on UUID).  In such
> > cases, instead of passing a single HOB, the boot phases may rely on passing
> > the pointer to the HOB table.
> >
> >
> >
> > This has been extremely useful concept to employ on highly configurable
> > systems that must rely on flexible discovery mechanisms to initialize and
> > boot the system.  This is especially helpful when you have multiple
> >
> >
> >
> >
> >
> > Why do we need HOBs in TF-A?:
> >
> > -----------------------------
> >
> > It is desirable that EL3 firmware (e.g. TF-A) built for ARM Server SoC in
> > a way that is SoC specific *but* platform agnostic.  This means that a
> > single ARM SoC that a SiP may deliver to customers may provide a single
> > TF-A binary (e.g. BL1, BL2, BL31) that could be used to support a broad
> > range of platform designs and configurations in order to boot a platform
> > specific firmware (e.g. BL33 and possibly even BL32 code).  In order to
> > achieve this, the platform configuration must be *discovered* instead of
> > statically compiled as it is today in TF-A via device tree based
> > enumeration.  The mechanisms of discovery may differ broadly depending on
> > the relevant industry standard, or in some cases may have rely on SiP
> > specific discovery flows.
> >
> >
> >
> > For example:  On server systems that support a broad range DIMM memory
> > population/topologies, all the necessary information required to boot is
> > fully discovered via standard JEDEC Serial Presence Detect (SPD) over an
> > I2C bus.  Leveraging the SPD bus, may platform variants could be supported
> > with a single TF-A binary.  Not only is this information required to
> > initialize memory in early boot phases (e.g. BL2), the subsequent boot
> > phases will also need this SPD info to construct a system physical address
> > map and properly initialize the MMU based on the memory present, and where
> > the memory may be present.  Subsequent boot phases (e.g. BL33 / UEFI) may
> > need to generate standard firmware tables to the operating systems, such as
> > SMBIOS tables describing DIMM topology and various ACPI tables (e.g. SLIT,
> > SRAT, even NFIT if NVDIMM's are present).
> >
> >
> >
> > In short, it all starts with a standardized or vendor specific discovery
> > flow in an early boot stage (e.g. BL1/BL2), followed by the passing of
> > information to the next boot stages (e.g. BL31/BL32/BL33).
> >
> >
> >
> > Today, every HOB may be a vendor specific structure, but in the future
> > there may be benefit of defining standard HOBs.  This may be useful for
> > memory discovery, passing the system physical address map, enabling TPM
> > measured boot, and potentially many other common HOB use-cases.
> >
> >
> >
> > It would be extremely beneficial to the datacenter market segment if the
> > TF-A community would adopt this concept of information passing between all
> > boot phases as opposed to rely solely on device tree enumeration.  This is
> > not intended to replace device tree, rather intended as an alternative way
> > to describe the info that must be discovered and dynamically generated.
> >
> >
> >
> >
> >
> > Conclusion:
> >
> > -----------
> >
> > We are proposing that the TF-A community begin pursuing the adoption of
> > HOBs as a mechanism used for information exchange between each boot stage
> > (e.g. BL1->BL2, BL2->BL31, BL31->BL32, and BL31->BL33)?  Longer term we
> > want to explore standardizing some HOB structures for the BL33 phase (e.g.
> > UEFI HOB structures), but initially would like to agree on this being a
> > useful mechanism used to pass information between each boot stage.
> >
> >
> >
> > Thanks,
> >
> > --Harb
> >
> >
> >
> >
> >
> >
> > --
> > TF-A mailing list
> > TF-A at lists.trustedfirmware.org<mailto:TF-A at lists.trustedfirmware.org>
> > https://lists.trustedfirmware.org/mailman/listinfo/tf-a
> >
> --
> François-Frédéric Ozog | *Director Linaro Edge & Fog Computing Group*
> T: +33.67221.6485
> francois.ozog at linaro.org<mailto:francois.ozog at linaro.org> | Skype: ffozog
> _______________________________________________
> boot-architecture mailing list
> boot-architecture at lists.linaro.org<mailto:boot-architecture at lists.linaro.org>
> https://lists.linaro.org/mailman/listinfo/boot-architecture



François-Frédéric Ozog | Director Linaro Edge & Fog Computing Group

T: +33.67221.6485
francois.ozog at linaro.org<mailto:francois.ozog at linaro.org> | Skype: ffozog

TF-A mailing list
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François-Frédéric Ozog | Director Linaro Edge & Fog Computing Group

T: +33.67221.6485
francois.ozog at linaro.org<mailto:francois.ozog at linaro.org> | Skype: ffozog

TF-A mailing list
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