Question regarding "boot-hartid" DT node
Jessica Clarke
jrtc27 at jrtc27.com
Sun Dec 5 07:54:23 PST 2021
On 5 Dec 2021, at 13:39, Sunil V L <sunilvl at ventanamicro.com> wrote:
>
> On Sat, Dec 04, 2021 at 10:34:28AM -0800, Atish Patra wrote:
>> On Fri, Dec 3, 2021 at 8:24 PM Anup Patel <anup at brainfault.org> wrote:
>>>
>>> On Sat, Dec 4, 2021 at 7:17 AM Heinrich Schuchardt
>>> <heinrich.schuchardt at canonical.com> wrote:
>>>>
>>>>
>>>>
>>>> On 12/4/21 01:44, Atish Patra wrote:
>>>>> On Fri, Dec 3, 2021 at 10:45 AM Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>>>>>>
>>>>>> On 12/3/21 11:53 AM, Heinrich Schuchardt wrote:
>>>>>>> On 12/3/21 11:13, Ard Biesheuvel wrote:
>>>>>>>> On Thu, 2 Dec 2021 at 20:29, Atish Patra <atishp at atishpatra.org> wrote:
>>>>>>>>>
>>>>>>>>> On Thu, Dec 2, 2021 at 9:05 AM Heinrich Schuchardt
>>>>>>>>> <heinrich.schuchardt at canonical.com> wrote:
>>>>>>>>>>
>>>>>>>>>> On 12/2/21 17:58, Ard Biesheuvel wrote:
>>>>>>>>>>> On Thu, 2 Dec 2021 at 17:53, Heinrich Schuchardt
>>>>>>>>>>> <heinrich.schuchardt at canonical.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> On 12/2/21 17:20, Ard Biesheuvel wrote:
>>>>>>>>>>>>> On Thu, 2 Dec 2021 at 16:05, Sunil V L <sunilvl at ventanamicro.com>
>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Hi All,
>>>>>>>>>>>>>> I am starting this thread to discuss about the
>>>>>>>>>>>>>> "boot-hartid" DT node
>>>>>>>>>>>>>> that is being used in RISC-V Linux EFI stub.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> As you know, the boot Hart ID is passed in a0 register to
>>>>>>>>>>>>>> the kernel
>>>>>>>>>>>>>> and hence there is actually no need to pass it via DT.
>>>>>>>>>>>>>> However, since
>>>>>>>>>>>>>> EFI stub follows EFI application calling conventions, it
>>>>>>>>>>>>>> needs to
>>>>>>>>>>>>>> know the boot Hart ID so that it can pass it to the proper
>>>>>>>>>>>>>> kernel via
>>>>>>>>>>>>>> a0. For this issue, the solution was to add
>>>>>>>>>>>>>> "/chosen/boot-hartid" in
>>>>>>>>>>>>>> DT. Both EDK2 and u-boot append this node in DT.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> I think this was a mistake tbh
>>>>>>>>>>>>>
>>>>>>>>>>>>>> But above approach causes issue for ACPI since ACPI
>>>>>>>>>>>>>> initialization
>>>>>>>>>>>>>> happens late in the proper kernel. Same is true even if we
>>>>>>>>>>>>>> pass this
>>>>>>>>>>>>>> information via SMBIOS.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> It would be better to define a RISCV specific EFI protocol that the
>>>>>>>>>>>>> stub can call to discover the boot-hartid value. That wat, it can
>>>>>>>>>>>>> pass
>>>>>>>>>>>>> it directly, without having to rely on firmware tables.
>>>>>>>>>>>>
>>>>>>>>>>>> As discovering the current process' hartid is not a UEFI specific
>>>>>>>>>>>> task
>>>>>>>>>>>> SBI would be a better fit.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> I disagree. The OS<->loader firmware interface is UEFI not SBI. So if
>>>>>>>>>>> the EFI stub needs to ask the firmware which boot-hartid it should
>>>>>>>>>>> pass in A0, it should use a EFI protocol and nothing else.
>>>>>>>>>>>
>>>>>>>>>>> Whether or not the loader/firmware *implements* this EFI protocol by
>>>>>>>>>>> calling into SBI is a different matter (and likely the best choice).
>>>>>>>>>>> But that does not mean the EFI stub should make SBI calls directly.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> The EFI stub does not need the boot-hartid. It is the main Linux kernel
>>>>>>>>>> that does. And that kernel already implements SBI calls.
>>>>>>>>>>
>>>>>>>>>> The main kernel could just try to read CSR mhartid which fails in
>>>>>>>>>> S-mode
>>>>>>>>>> and the SBI could emulate it.
>>>>>>>>>
>>>>>>>>> New SBI extension should be added only if there is no other way to
>>>>>>>>> solve a generic
>>>>>>>
>>>>>>> I am not sure this feature would be implemented as SBI extension or as a
>>>>>>> CSR emulation. Cf. sbi_emulate_csr_read(). But anyway it would require
>>>>>>> an update of the SBI specification.
>>>>>>>
>>>>>>>>> problem. The boot hartid issue is very specific to efi booting only.
>>>>>>>>> Any system that doesn't require
>>>>>>>
>>>>>>> The boot hartid is not EFI related at all. A firmware running single
>>>>>>> threaded does not need this information.
>>>>>>>
>>>>>>> Information about the boot hartid is a general OS need.
>>>>>>>
>>>>>>> I am wondering why S-mode software should not have a generic means to
>>>>>>> find out on which hart it is currently running.
>>>>>>>
>>>>>>>>> EFI booting won't need it. Even for EFI booting, we have other
>>>>>>>>> approaches already proposed
>>>>>>>>> to solve it. That's why, SBI extension should be the last resort
>>>>>>>>> rather than first.
>>>>>>>>>
>>>>>>>>> I think an RISC-V specific EFI protocol as suggested by Ard should
>>>>>>>>> work for all the cases.
>>>>>>>>> Is there a case where you think it may not work ? U-Boot & EDK2
>>>>>>>>> already stores the boot hartid.
>>>>>>>>> They just implement that protocol and pass the hartid to the caller.
>>>>>>>>> We do need to support it in the grub though.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Why would GRUB care about this? The EFI stub will call into the
>>>>>>>> underlying firmware to invoke the protocol, GRUB is just a loader with
>>>>>>>> a fancy menu that allows you to select which image to load, no?
>>>>>>>
>>>>>>> This is a related discussion:
>>>>>>>
>>>>>>> https://github.com/tekkamanninja/grub/commit/be9d4f1863a1fcb1cbbd2f867309457fade8be73#commitcomment-60851029
>>>>>>>
>>>>>
>>>>> Yes!! Thanks for refreshing the memory. It seems after 2 years, we are
>>>>> still debating the same topic :).
>>>>> Let me summarize the thread. There are multiple ways for EFI stub code
>>>>> to retrieve the boot hartid.
>>>>>
>>>>> 1. EFI variables - This is what Henerich proposed last time. Ard
>>>>> suggested that EFI configuration tables are better candidates than EFI
>>>>> variables.
>>>>> 2. DT modification - This was preferred over the configuration table
>>>>> at that time given because RISC-V was DT only at that time.
>>>>> We already had all the infrastructure
>>>>> around DT. Thus, DT seemed to be a natural choice then.
>>>>> It works now for existing setup
>>>>> however, the DT approach will not work for systems with ACPI support.
>>>>> Adding a similar entry in ACPI tables
>>>>> is possible but adding ACPI support in EFI stub is not trivial.
>>>>> 3. SMBIOS - Only for platforms with SMBIOS support. SMBIOS is not
>>>>> mandatory and adding SMBIOS support in EFI stub is not trivial.
>>>>> 4. SBI - As I mentioned before, this is an EFI specific
>>>>> problem because EFI stub doesn't know what the boot hartid is. Thus,
>>>>> it should be solved
>>>>> in an EFI specific way. An SBI extension for
>>>>> such features may not be acceptable as the non-EFI booting method
>>>>> works fine without the SBI extension.
>>>>> 5. RISC-V specific EFI configuration table or protocol: Ard suggested
>>>>> EFI configuration table last time. Earlier in this thread, EFI
>>>>> protocol was suggested.
>>>>> My personal preference has always been one of
>>>>> these as it solves the problem for all EFI booting methods
>>>>> for platforms-os
>>>>> combination(DT/ACPI-Linux/FreeBSD) in an EFI specific way.
>>>>>
>>>>> @Heinrich: Do you see any issue with the EFI configuration table or
>>>>> protocol to retrieve the boot hartid?
>>>>
>>>> There is nothing technical stopping us from implementing either option.
>>>>
>>>> We could simply reuse the EFI Device Tree Fixup Protocol
>>>> (https://github.com/U-Boot-EFI/EFI_DT_FIXUP_PROTOCOL) implemented in
>>>> U-Boot and already used by systemd-boot. Pass a devicetree (which may be
>>>> empty) to the Fixup() method and it will add the /chosen node with the
>>>> boot-hartid parameter.
>>>>
>>>> The EFI stub anyway creates a new device-tree to pass the memory map to
>>>> the kernel in the ACPI case (function update_fdt()). Calling the EFI
>>>> Device Tree Fixup Protocol could be easily added.
>>
>> Thanks. Yes. We can solve the current problem for EFI stub in Linux.
>>
>>>
>>> Are you suggesting that DTB (skeletal or full-blown) will always be there when
>>> booting the kernel as an EFI application ? If yes then we are
>>> indirectly mandating
>>> skeletal DTB for UEFI+ACPI system.
>>
>> Yes. EFI Stub tries to find a fdt from the command line (not a
>> preferred method) or EFI configuration table[1]
>> (currently used for RISC-V systems). If it can't find a device tree,
>> it generates one [2]
>>
>> [1] https://elixir.bootlin.com/linux/v5.16-rc3/source/drivers/firmware/efi/libstub/efi-stub.c#L231
>> [2] https://elixir.bootlin.com/linux/v5.16-rc3/source/drivers/firmware/efi/libstub/fdt.c#L58
>>
>> However, we may still need to define the RISC-V EFI protocol to
>> support ACPI for other OS (FreeBSD) which doesn't have
>> a stub like loader that uses DT.
>>
> If RISC-V mandates that every OS (including FreeBSD) should follow the
> booting protocol that a0 should be used to pass the boot hartid, then
> only Linux EFI stub becomes a special case which can reuse the existing
> DT based EFI protocol (mentioned by Heinrich), right?
Absolutely not. What you are saying is “let’s forbid anyone from using
UEFI”. This “booting protocol” thing is a RISC-V-specific low-level
interface that is incompatible with UEFI; in it, every architecture,
including RISC-V, passes exactly two arguments to the application, the
handle to the image and a pointer to the system table, and puts the
return address in the usual place. Linux’s EFI stub is not special;
*any* EFI application, be it your EFI stub, FreeBSD’s bootloader,
grub-efi, systemd-boot, OpenBSD’s bootloader, Haiku’s bootloader,
whatever, follows the UEFI spec and so does not, and never will, follow
the RISC-V booting protocol.
I wish everyone would stop talking about this “RISC-V booting
protocol”. It’s low-level firmware details that, due to not having a
mature firmware story at the time OSes were brought up, leaked up into
the OS booting process. In FreeBSD the only time it gets used is if the
kernel is booted directly from BBL/OpenSBI; if instead you boot the
bootloader via UEFI then that code is completely bypassed (and we make
the legacy direct boot method look like the bootloader method by
creating a “fake” bootloader metadata array, rather than the other way
round, which would be backwards and severely limiting). It’s not a good
protocol, OSes shouldn’t be using it, and it should fade into obscurity
outside of the M-mode firmware<->S-mode firmware interface. It’s just
not relevant to an OS. What is relevant is that there is critical
information in the firmware that the OS can’t get at if booting via the
industry-standard portable rich firmware interface, and that
information needs to be provided some way or another via a means
compatible with the specification. I like the SBI “get current hartid”
extension approach best, it’s the most flexible and there’s no harm in
having it exist in a crummy non-UEFI situation. It’s trivial to
implement, it’s trivial to use, it provides the interface that *should*
have existed in the ISA just like how mpidr_el1 is a thing on AArch64,
and it provides a way for even non-UEFI code to not have to care about
a0 being hartid (outside of perhaps the hart lottery in the entry point
if you need to support pre-HSM SBIs), it can just throw it away and ask
for it later when it needs it, just like if booting via UEFI. You could
also make it a UEFI protocol, but that’s more annoying to deal with,
you now have two different ways to get the same information depending
on how you were booted. You could also restrict it to only being “get
the boot hartid”, but why, what does that achieve? It doesn’t really
make it any simpler to implement, it’s just more restrictive for the
sake of being minimal, even if it is sufficient. But at the end of the
day all of them do work. So, please, can we stop wasting our lives on
this and just do *something* rather than going round and round in
circles and forgetting all the details in the process?
As for the “provide a dummy FDT to the proposed fixup protocol”
proposal, no, that’s clearly not going to fly with an ACPI-only OS that
doesn’t have, and never will include, a device tree parser. If your
ACPI system needs a device tree then it’s not ACPI-only any more.
Jess
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