Message-ID: <aLsOt5Tr+ThuHmSS@e133380.arm.com>
Date: Fri, 5 Sep 2025 17:24:23 +0100
From: Dave Martin <Dave.Martin@....com>
To: James Morse <james.morse@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH 03/33] ACPI / PPTT: Add a helper to fill a cpumask from a
 processor container

Hi James,

On Thu, Aug 28, 2025 at 04:57:06PM +0100, James Morse wrote:
> Hi Dave,
> 
> On 27/08/2025 11:48, Dave Martin wrote:
> > On Fri, Aug 22, 2025 at 03:29:44PM +0000, James Morse wrote:
> >> The PPTT describes CPUs and caches, as well as processor containers.
> >> The ACPI table for MPAM describes the set of CPUs that can access an MSC
> >> with the UID of a processor container.
> >>
> >> Add a helper to find the processor container by its id, then walk
> >> the possible CPUs to fill a cpumask with the CPUs that have this
> >> processor container as a parent.
> 
> > Nit: The motivation for the change is not clear here.
> > 
> > I guess this boils down to the need to map the MSC topology information
> > in the the ACPI MPAM table to a cpumask for each MSC.
> > 
> > If so, a possible rearrangement and rewording might be, say:
> > 
> > --8<--
> > 
> > The ACPI MPAM table uses the UID of a processor container specified in
> > the PPTT, to indicate the subset of CPUs and upstream cache topology
> > that can access each MPAM Memory System Component (MSC).
> > 
> > This information is not directly useful to the kernel.  The equivalent
> > cpumask is needed instead.
> > 
> > Add a helper to find the processor container by its id, then [...]
> > 
> > -->8--
> 
> Thanks, that is clearer!

Thanks

> >> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c

[...]

> >> @@ -298,6 +298,92 @@ static struct acpi_pptt_processor *acpi_find_processor_node(struct acpi_table_he

[...]

> >> +static void acpi_pptt_get_child_cpus(struct acpi_table_header *table_hdr,
> >> +				     struct acpi_pptt_processor *parent_node,
> >> +				     cpumask_t *cpus)
> >> +{
> >> +	struct acpi_pptt_processor *cpu_node;
> >> +	u32 acpi_id;
> >> +	int cpu;
> >> +
> >> +	cpumask_clear(cpus);
> >> +
> >> +	for_each_possible_cpu(cpu) {
> >> +		acpi_id = get_acpi_id_for_cpu(cpu);
> 
> > ^ Presumably this can't fail?
> 
> It'll return something! This could only be a problem if this raced with a CPU becoming
> impossible, and there is no mechanism to do that.

Yep, now I go and look more closely at that function, my question looks
misguided.

[...]

> >> +void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus)
> >> +{
> >> +	struct acpi_pptt_processor *cpu_node;
> >> +	struct acpi_table_header *table_hdr;
> >> +	struct acpi_subtable_header *entry;
> >> +	unsigned long table_end;
> >> +	acpi_status status;
> >> +	bool leaf_flag;
> >> +	u32 proc_sz;
> >> +
> >> +	cpumask_clear(cpus);
> >> +
> >> +	status = acpi_get_table(ACPI_SIG_PPTT, 0, &table_hdr);
> >> +	if (ACPI_FAILURE(status))
> >> +		return;
> 
> > Is acpi_get_pptt() applicable here?
> 
> Oh, that is new, and would let me chuck the reference counting.
> I guess this replaces Jonthan's magic table free'ing cleanup thing!

Ah, rightho.

> > (That function is not thread-safe, but then, perhaps most/all of these
> > functions are not thread safe.  If we are still on the boot CPU at this
> > point (?) then this wouldn't be a concern.)
> 
> I think that relies on the first caller being from somewhere that can't race.
> In this case its the architecture's smp_prepare_cpus() call to setup the acpi topology.
> That is sufficiently early its not a concern.

I guess so.

[...]

> >> +		cpu_node = (struct acpi_pptt_processor *)entry;
> >> +		if (entry->type == ACPI_PPTT_TYPE_PROCESSOR &&
> >> +		    cpu_node->flags & ACPI_PPTT_ACPI_PROCESSOR_ID_VALID) {
> >> +			leaf_flag = acpi_pptt_leaf_node(table_hdr, cpu_node);
> >> +			if (!leaf_flag) {
> >> +				if (cpu_node->acpi_processor_id == acpi_cpu_id)
> 
> 
> > Is there any need to distinguish processor containers from (leaf) CPU
> > nodes, here?  If not, dropping the distinction might simplify the code
> > here (even if callers do not care).
> 
> In the namespace the object types are different, so I assumed they have their own UID
> space. The PPTT holds both - hence the check for which kind of thing it is. The risk is
> looking for processor-container-4 and finding CPU-4 instead...
>
> The relevant ACPI bit is "8.4.2.1 Processor Container Device", its says:
> | A processor container declaration must supply a _UID method returning an ID that is
> | unique in the processor container hierarchy.
> 
> Which doesn't quite let me combine them here.

I was going by the PPTT spec, where the types are not distinct --
you're probably right, though.

According to that, isn't it the "ACPI Processor ID valid" flag, not the
"Node is a Leaf" flag, that says whether this field is meaningful?

It's reasonable not to bother to try to enumerate the children of a
node that claims to be a leaf (even if there actually are children),
but I wonder what happens if acpi_processor_id is not declared to be
valid and matches by accident.  That's probably not a valid table (?)
but does anything bad happen on the kernel side?

> > Otherwise, maybe eliminate leaf_flag and collapse these into a single
> > if(), as suggested by Ben [1].
> > 
> >> +					acpi_pptt_get_child_cpus(table_hdr, cpu_node, cpus);
> > 
> > Can there ever be multiple matches?
> > 
> > The possibility of duplicate processor IDs in the PPTT sounds weird to
> > me, but then I'm not an ACPI expert.
> 
> Multiple processor-containers with the same ID? That would be a corrupt table.
> acpi_pptt_get_child_cpus() then walks the tree again to find the CPUs below this
> processor-container - those have a different kind of id.

Does anything bad happen if we encounter duplicates?

(Other then the MPAM driver never getting enabled, or not working as
advertised, that is.)

I haven't tried to think through all the implications, here.

> > If there can only be a single match, though, then we may as well break
> > out of the loop here, unless we want to be paranoid and report
> > duplicates as an error -- but that would require extra implementation,
> > so I'm not sure that would be worth it.
> 
> Hmmm, the PPTT node should map to only one processor or processor-container.
> I'll chuck the break in.

Ack

Cheers
---Dave

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