Re: [PATCH v7 2/2] schemas: Add some common reserved-memory usages

[Ard Biesheuvel]

On Fri, 22 Dec 2023 at 20:52, Chiu, Chasel <chasel.chiu@xxxxxxxxx> wrote:
>
>
> Please see my reply below inline.
>
> Thanks,
> Chasel
>
...
> > > > The gEfiMemoryTypeInformationGuid HOB typically carries platform
> > > > defaults, and the actual memory type information is kept in a
> > > > non-volatile EFI variable, which gets updated when the memory usage
> > > > changes. Is this different for UefiPayloadPkg?
> > > >
> > > > (For those among the cc'ees less versed in EFI/EDK2: when you get
> > > > the 'config changed -rebooting' message from the boot firmware, it
> > > > typically means that this memory type table has changed, and a
> > > > reboot is necessary.)
> > > >
> > > > So the platform init needs to read this variable, or get the
> > > > information in a different way. I assume it is the payload, not the
> > > > platform init that updates the variable when necessary. This means
> > > > the information flows from payload(n) to platform init(n+1), where n
> > > > is a monotonic index tracking consecutive boots of the system.
> > > >
> > > > Can you explain how the DT fits into this? How are the runtime-code
> > > > and runtime-data memory reservation nodes under /reserved-memory
> > > > used to implement this information exchange between platform init
> > > > and payload? And how do the HOB and the EFI variable fit into this picture?
> > >
> > >
> > > 1. With some offline discussion, we would move
> > > gEfiMemoryTypeInformationGuid usage to FDT->upl-custom node. This is
> > > because it is edk2 implementation choice and non-edk2 PlatformInit or
> > > Payload may not have such memory optimization implementation. (not a
> > > generic usage/requirement for PlatformInit and Payload)
> > >
> > > The edk2 example flow will be like below:
> > >
> > > PlatformInit to GetVariable of gEfiMemoryTypeInformationGuid and create Hob-
> > >
> > >   PlatformInit to initialize FDT->upl-custom node to report
> > gEfiMemoryTypeInformationGuid HOB information ->
> > >     UefiPayload entry to re-create gEfiMemoryTypeInformationGuid HOB basing
> > on FDT input (instead of the default MemoryType inside UefiPayload) ->
> > >       UefiPayload DxeMain/Gcd will consume gEfiMemoryTypeInformationGuid
> > Hob for memory type information ->
> > >         UefiPayload to initialize UEFI environment (mainly DXE dispatcher) ->
> > >           (additional FV binary appended to common UefiPayload binary)
> > PlatformPayload to provide VariableService which is platform specific ->
> > >             UefiPayload UefiBootManager will SetVariable if memory type change
> > needed and request a warm reset ->
> > >               Back to PlatformInit ...
> > >
> >
> > OK so the upl-custom node can do whatever it needs to. I imagine these will
> > include the memory descriptor attribute field, and other parts that may be missing
> > from the /reserved-memory DT node specification?
>
>
> Yes, if needed by edk2 specific implementation, not generic enough, we may consider to use upl-custom node to pass those data.
>
>
> >
> > >
> > > 2. Now the proposed reserved-memory node usages will be for PlatformInit to
> > provide data which may be used by Payload or OS. This is not edk2 specific and
> > any PlatformInit/Payload could have same support.
> > > Note: all of below are optional and PlatformInit may choose to implement some
> > of them or not.
> > >
> > >       - acpi
> > > If PlatformInit created some ACPI tables, this will report a memory region which
> > contains all the tables to Payload and Payload may base on this to add some more
> > tables if required.
> > >
> > >       - acpi-nvs
> > > If PlatformInit has created some ACPI tables which having ACPI NVS memory
> > dependency, this will be that nvs region.
> > >
> >
> > These make sense.
> >
> > >       - boot-code
> > > When PlatformInit having some FW boot phase code that could be freed
> > > for OS to use when payload transferring control to UEFI OS
> > >
> > >       - boot-data
> > > When PlatformInit having some FW boot phase data that could be freed for OS
> > to use when payload transferring control to UEFI OS.
> > >
> > >       - runtime-code
> > > PlatformInit may provide some services code that can be used for Payload to
> > initialize UEFI Runtime Services for supporting UEFI OS.
> > >
> > >       - runtime-data
> > > PlatformInit may provide some services data that can be used for Payload to
> > Initialize UEFI Runtime Services for supporting UEFI OS.
> > >
> >
> > A UEFI OS must consume this information from the UEFI memory map, not from
> > the /reserved-memory nodes. So these nodes must either not be visible to the OS
> > at all, or carry an annotation that the OS must ignore them.
> >
> > Would it be possible to include a restriction in the DT schema that these are only
> > valid in the firmware boot phase?
>
>
> https://uefi.org/specs/UEFI/2.10/07_Services_Boot_Services.html#efi-boot-services-exitbootservices
> Per UEFI specification, UEFI OS will always call UEFI GetMemoryMap function to retrieve memory map, so FDT node present or not does not matter to UEFI OS. We probably could have annotation in UPL specification to emphasize this.
> I'm not familiar with Linux FDT boot, but if non-UEFI OS does not call UEFI GetMemoryMap() and does not know what is runtime-code/data, boot-code/data, it might just treat such reserved-memory nodes as 'regular' reserved memory nodes, and that's still ok because non-UEFI OS will not call to any runtime service or re-purpose boot-code/data memory regions.
>

You are saying the same thing but in a different way. A UEFI OS must
only rely on GetMemoryMap(), and not on the /reserved-memory node to
obtain this information. But this requirement needs to be stated
somewhere: the UEFI spec does not reason about other sources of EFI
memory information at all, and this DT schema does not mention any of
this either.

> Would you provide a real OS case which will be impacted by this reserved-memory schema so we can discuss basing on real case?
>

Funny, that is what I have been trying to get from you :-)

The problem I am anticipating here is that the information in
/reserved-memory may be out of sync with the EFI memory map. It needs
to be made clear that the EFI memory map is the only source of truth
when the OS is involved, and this /reserved-memory mechanism should
only be used by other firmware stages. But the schema does not mention
this at all. The schema also does not mention that the information in
/reserved-memory is not actually sufficient to reconstruct the EFI
memory map that the firmware payload expects (which is why the
upl-custom-node exists too)