Re: [PATCH v5 3/4] schemas: Add some common reserved-memory usages

[Ard Biesheuvel]

On Wed, 6 Sept 2023 at 16:54, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
>
> Hi Rob, Ard,
>
> On Wed, 6 Sept 2023 at 08:34, Rob Herring <robh@xxxxxxxxxx> wrote:
> >
> > On Tue, Sep 5, 2023 at 4:44 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > >
> > > On Thu, 31 Aug 2023 at 01:18, Simon Glass <sjg@xxxxxxxxxxxx> 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.
> > > >
> > >
> > > I don't think the UEFI references are needed or helpful here.
> > >
> > > > This aims to provide an small schema addition for this mapping.
> > > >
> > > > For now, no attempt is made to create an exhaustive binding, so there are
> > > > some example types listed. More can be added later.
> > > >
> > > > The compatible string is not included, since the node name is enough to
> > > > indicate the purpose of a node, as per the existing reserved-memory
> > > > schema.
> >
> > Node names reflect the 'class', but not what's specifically in the
> > node. So really, all reserved-memory nodes should have the same name,
> > but that ship already sailed for existing users. 'compatible' is the
> > right thing here. As to what the node name should be, well, we haven't
> > defined that. I think we just used 'memory' on some platforms.
>
> OK
>
> >
> > > > This binding does not include a binding for the memory 'attribute'
> > > > property, defined by EFI_BOOT_SERVICES.GetMemoryMap(). It may be useful
> > > > to have that as well, but perhaps not as a bit mask.
> > > >
> > > > Signed-off-by: Simon Glass <sjg@xxxxxxxxxxxx>
> > > > ---
> > > >
> > > > Changes in v5:
> > > > - Drop the memory-map node (should have done that in v4)
> > > > - Tidy up schema a bit
> > > >
> > > > Changes in v4:
> > > > - Make use of the reserved-memory node instead of creating a new one
> > > >
> > > > Changes in v3:
> > > > - Reword commit message again
> > > > - cc a lot more people, from the FFI patch
> > > > - Split out the attributes into the /memory nodes
> > > >
> > > > Changes in v2:
> > > > - Reword commit message
> > > >
> > > >  .../reserved-memory/common-reserved.yaml      | 53 +++++++++++++++++++
> > > >  1 file changed, 53 insertions(+)
> > > >  create mode 100644 dtschema/schemas/reserved-memory/common-reserved.yaml
> > > >
> > > > diff --git a/dtschema/schemas/reserved-memory/common-reserved.yaml b/dtschema/schemas/reserved-memory/common-reserved.yaml
> > > > new file mode 100644
> > > > index 0000000..d1b466b
> > > > --- /dev/null
> > > > +++ b/dtschema/schemas/reserved-memory/common-reserved.yaml
> > > > @@ -0,0 +1,53 @@
> > > > +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> > > > +%YAML 1.2
> > > > +---
> > > > +$id: http://devicetree.org/schemas/reserved-memory/common-reserved.yaml#
> > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > +
> > > > +title: Common memory reservations
> > > > +
> > > > +description: |
> > > > +  Specifies that the reserved memory region can be used for the purpose
> > > > +  indicated by its node name.
> > > > +
> > > > +  Clients may reuse this reserved memory if they understand what it is for.
> > > > +
> > > > +maintainers:
> > > > +  - Simon Glass <sjg@xxxxxxxxxxxx>
> > > > +
> > > > +allOf:
> > > > +  - $ref: reserved-memory.yaml
> > > > +
> > > > +properties:
> > > > +  $nodename:
> > > > +    enum:
> > > > +      - acpi-reclaim
> > > > +      - acpi-nvs
> > > > +      - boot-code
> > > > +      - boot-data
> > > > +      - runtime-code
> > > > +      - runtime-data
> > > > +
> > >
> > > These types are used by firmware to describe the nature of certain
> > > memory regions to the OS. Boot code and data can be discarded, as well
> > > as ACPI reclaim after its contents have been consumed. Runtime code
> > > and data need to be mapped for runtime features to work.
> > >
> > > When one firmware phase communicates the purpose of a certain memory
> > > reservation to another, it is typically not limited to whether its
> > > needs to be preserved and when it needs to be mapped (and with which
> > > attributes). I'd expect a memory reservation appearing under this node
> > > to have a clearly defined purpose, and the subsequent phases need to
> > > be able to discover this information.
> > >
> > > For example, a communication buffer for secure<->non-secure
> > > communication or a page with spin tables used by PSCI. None of the
> > > proposed labels are appropriate for this, and I'd much rather have a
> > > compatible string or some other property that clarifies the nature in
> > > a more suitable way. Note that 'no-map' already exists to indicate
> > > that the CPU should not map this memory unless it does so for the
> > > specific purpose that the reservation was made for.
> >
> > I agree. I think compatible is the better approach. Some property like
> > 'discard' may not be sufficient information if the OS needs to consume
> > the region first and then discard it. Better to state exactly what's
> > there and then the OS can imply the rest.
>
> OK, so what sort of compatible strings?
>
> How about:
> "acpi-reclaim" - holds ACPI tables; memory can be reclaimed once the
> tables are read and no-longer needed

ACPI reclaim is a policy, not a purpose. This memory could contain
many different things.

> "boot-code" - holds boot code; memory can be reclaimed once the boot
> phase is complete
> "runtime-code" - holds runtime code; memory can be reclaimed only if
> this code will not be used from that point
>

These are also policies. They can be inferred from the purpose.

> etc. We can then have more specific compatibles, like:
>
> "psci-spin-table" - holds PSCI spin tables
>
> so you could do:
>
> compatible = "runtime-code", "psci-spin-table";
>

I understand that this binding targets firmware<->firmware rather than
firmware<->OS, which makes it much more difficult to keep it both
generic and sufficiently descriptive.

However, I still feel that all the overlap with UEFI memory types is
not what we want here. UEFI knows how to manage its own memory map,
what it needs to know is what memory is already in use and for which
exact purpose. Whether or not that implies that the memory can be
freed at some point or can be mapped or not should follow from that.