Re: [PATCH] sparc: populate sysfs cache topology information on sun4v
From: Tony Rodriguez
Date: Sun Oct 18 2026 - 20:18:08 EST
Hi Jan,
On 10/18/26 3:27 PM, Jan Engelhardt wrote:
The SPARC port does not publish any cache topology information. The
lscpu utility therefore falls back to assuming the published cache
sizes are for unshared caches, and as a result, reports way more
cache than there really is present in the system. On a T5240 machine:
```
$ lscpu
Architecture: sparc64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Big Endian
CPU(s): 128
On-line CPU(s) list: 0-127
Model name: UltraSparc T2 (Niagara2)
Thread(s) per core: 4
Core(s) per socket: 16
Socket(s): 2
Flags: sun4v
Caches (sum of all):
L1d: 1 MiB (128 instances)
L1i: 2 MiB (128 instances)
L2: 512 MiB (128 instances)
```
This patch implements population of the sysfs cacheinfo structures
for sun4v (systems with mdesc). The proper amount is now reflected in
lscpu's output:
```
Caches (sum of all):
L1d: 128 KiB (16 instances)
L1i: 256 KiB (16 instances)
L2: 8 MiB (2 instances)
```
Signed-off-by: Jan Engelhardt <jengelh@xxxxxxx>
---
arch/sparc/include/asm/cpudata_64.h | 8 +++-
arch/sparc/kernel/mdesc.c | 61 +++++++++++++++++++++++++++++
2 files changed, 68 insertions(+), 1 deletion(-)
diff --git a/arch/sparc/include/asm/cpudata_64.h b/arch/sparc/include/asm/cpudata_64.h
index 056b3c0e7ef9..e28b841ffe23 100644
--- a/arch/sparc/include/asm/cpudata_64.h
+++ b/arch/sparc/include/asm/cpudata_64.h
@@ -18,13 +18,19 @@ typedef struct {
unsigned int irq0_irqs;
unsigned int __pad2;
- /* Dcache line 2, rarely used */
+ /* Dcache line 2+3, rarely used */
unsigned int dcache_size;
unsigned int dcache_line_size;
+ unsigned int dcache_assoc;
+ unsigned int dcache_id;
unsigned int icache_size;
unsigned int icache_line_size;
+ unsigned int icache_assoc;
+ unsigned int icache_id;
unsigned int ecache_size;
unsigned int ecache_line_size;
+ unsigned int ecache_assoc;
+ unsigned int ecache_id;
unsigned short sock_id; /* physical package */
unsigned short core_id;
unsigned short max_cache_id; /* groupings of highest shared cache */
diff --git a/arch/sparc/kernel/mdesc.c b/arch/sparc/kernel/mdesc.c
index 23b51a32e96e..029d9d812074 100644
--- a/arch/sparc/kernel/mdesc.c
+++ b/arch/sparc/kernel/mdesc.c
@@ -818,6 +818,7 @@ static void fill_in_one_cache(cpuinfo_sparc *c, struct mdesc_handle *hp, u64 mp)
const u64 *level = mdesc_get_property(hp, mp, "level", NULL);
const u64 *size = mdesc_get_property(hp, mp, "size", NULL);
const u64 *line_size = mdesc_get_property(hp, mp, "line-size", NULL);
+ const u64 *assoc = mdesc_get_property(hp, mp, "associativity", NULL);
const char *type;
int type_len;
@@ -828,15 +829,21 @@ static void fill_in_one_cache(cpuinfo_sparc *c, struct mdesc_handle *hp, u64 mp)
if (of_find_in_proplist(type, "instn", type_len)) {
c->icache_size = *size;
c->icache_line_size = *line_size;
+ c->icache_assoc = assoc ? *assoc : 0;
+ c->icache_id = mp;
} else if (of_find_in_proplist(type, "data", type_len)) {
c->dcache_size = *size;
c->dcache_line_size = *line_size;
+ c->dcache_assoc = assoc ? *assoc : 0;
+ c->dcache_id = mp;
}
break;
case 2:
c->ecache_size = *size;
c->ecache_line_size = *line_size;
+ c->ecache_assoc = assoc ? *assoc : 0;
+ c->ecache_id = mp;
break;
default:
@@ -1251,6 +1258,60 @@ void mdesc_fill_in_cpu_data(cpumask_t *mask)
smp_fill_in_sib_core_maps();
}
+int init_cache_level(unsigned int cpu)
+{
+ struct cpu_cacheinfo *i = get_cpu_cacheinfo(cpu);
+ i->num_levels = 2;
+ i->num_leaves = 3;
+ return 0;
+}
+
+int populate_cache_leaves(unsigned int cpu)
+{
+ struct cacheinfo *leaf = get_cpu_cacheinfo(cpu)->info_list;
+ cpuinfo_sparc *cs = &cpu_data(cpu);
+
+ leaf->id = cs->icache_id;
+ leaf->attributes = CACHE_ID;
+ leaf->level = 1;
+ leaf->type = CACHE_TYPE_INST;
+ leaf->coherency_line_size = cs->icache_line_size;
+ leaf->ways_of_associativity = cs->icache_assoc;
+ leaf->size = cs->icache_size;
+ leaf->priv = cs;
+ if (cs->icache_line_size > 0 && cs->icache_assoc > 0)
+ leaf->number_of_sets = cs->icache_size / cs->icache_line_size /
+ cs->icache_assoc;
+ ++leaf;
+
+ leaf->id = cs->dcache_id;
+ leaf->attributes = CACHE_ID;
+ leaf->level = 1;
+ leaf->type = CACHE_TYPE_DATA;
+ leaf->coherency_line_size = cs->dcache_line_size;
+ leaf->ways_of_associativity = cs->dcache_assoc;
+ leaf->size = cs->dcache_size;
+ leaf->priv = cs;
+ if (cs->dcache_line_size > 0 && cs->dcache_assoc > 0)
+ leaf->number_of_sets = cs->dcache_size / cs->dcache_line_size /
+ cs->dcache_assoc;
+ ++leaf;
+
+ leaf->id = cs->ecache_id;
+ leaf->attributes = CACHE_ID;
+ leaf->level = 2;
+ leaf->type = CACHE_TYPE_UNIFIED;
+ leaf->coherency_line_size = cs->ecache_line_size;
+ leaf->ways_of_associativity = cs->ecache_assoc;
+ leaf->size = cs->ecache_size;
+ leaf->priv = cs;
+ if (cs->ecache_line_size > 0 && cs->ecache_assoc > 0)
+ leaf->number_of_sets = cs->ecache_size / cs->ecache_line_size /
+ cs->ecache_assoc;
+
+ return 0;
+}
+
/* mdesc_open() - Grab a reference to mdesc_handle when /dev/mdesc is
* opened. Hold this reference until /dev/mdesc is closed to ensure
* mdesc data structure is not released underneath us. Store the
Thanks for working on this — publishing proper SPARC cache topology is long overdue, and reading associativity from MDESC plus carrying cache IDs through cpuinfo_sparc is a sensible direction.
*Full disclosure: I ran this patch through "DeepSeek AI" out of curiosity to sanity-check it and I am interested in learning more. Deep Seek AI flagged a few issues that seem worth mentioning: You may want to double check the "Deep Seek AI" review of this patch.
Main blockers
1) leaf->attributes = CACHE_ID is invalid. attributes is a bitmask of CACHE_WRITE_BACK / CACHE_WRITE_THROUGH / etc.; CACHE_ID isn’t a valid attribute. It should be 0 or real attribute bits from MDESC.
2) Shared-cache reporting is not actually wired up, so T2-class machines still show the wrong cache sizes. The patch sets leaf->id = mp and leaf->priv = cs, but never fills leaf->shared_cpu_map, and the generic detect_cache_attributes() -> cache_shared_cpu_map_setup() path doesn’t appear to be invoked on SPARC. As a result, the generic cacheinfo code still treats every leaf as private to one CPU, and lscpu still multiplies shared cache size by the number of threads — the exact bug being reported.
Concretely: with this patch applied, a T5240 would still report L2: 512 MiB (128 instances) rather than the correct L2: 8 MiB (2 instances) (4 MB per socket, shared across 64 threads, 2 sockets). A T5120 would still report the 4 MB shared L2 as 4 MB × 64 threads = 256 MiB in 64 instances , instead of 4 MiB (1 instance). The patch does not change the outcome on T2-class hardware because the leaves are still treated as private to each CPU.
3) MDESC level handling is incomplete and mislabels caches on S7/T7. fill_in_one_cache() only handles case 1: (L1I/L1D) and case 2: (ecache); everything else falls through to default:. On T2/T5240 that happens to work because the shared 4 MB is reported at level 2. But on S7-2/T7-1, MDESC reports L2 at level 2 and the shared L3 at level 3 — so the L2 gets stored as ecache_*, and the real L3 is silently ignored. cpuinfo_sparc also has no separate l2_* fields, so the data model can’t hold both levels.
4) Hardcoded num_levels = 2 / num_leaves = 3 and ecache labeled level 2 are wrong across the family. T2/T5240: L1I + L1D + shared L2 (no L3). S7-2: L1 + shared L2 + shared L3. T7-1: L1 + private L2 + shared L3. The leaf count, levels, and types need to be derived from MDESC, not hardcoded.
5) The new init_cache_level() / populate_cache_leaves() don’t appear to be wired into the generic cacheinfo path , so they may never actually be called.
Additional issues
6) leaf->id is int, but the patch stores a truncated u64 MDESC handle. ecache_id/ icache_id/ dcache_id are unsigned int, assigned from u64 mp. Beyond truncation, assigning u32 → int id can produce negative IDs. An index-based or hashed ID would be cleaner.
7) Growing cpuinfo_sparc needs an asm-offset audit. The struct has a “must be 1st” field and SPARC asm accesses cpu_data at fixed offsets in places like rtrap_64.S. The patch adds fields after that, so offset 0 is preserved, but it should be confirmed that no asm hardcodes offsets into the fields that now shift ( sock_id, core_id, max_cache_id, cache fields). It also grows the per-CPU area past the “Dcache line 2” region.
8) The data model has no separate L2 storage for S7/T7. cpuinfo_sparc only has ecache_* for the shared cache. To model S7 (shared L2 per cluster + shared L3 per socket) and T7 (private L2 per core + shared L3), you need distinct l2_* and l3/ ecache_* fields.
9) The new *_id scheme duplicates existing infrastructure. cpuinfo_sparc already has max_cache_id (“groupings of highest shared cache”) and smp_fill_in_sib_core_maps(). It’s worth checking whether that mechanism should be extended rather than adding six new fields.
10) num_leaves may not exist in the target kernel. struct cpu_cacheinfo gained num_leaves relatively late (~4.16). If the tree is older, init_cache_level() won’t compile or won’t allocate the right number of leaves.
11) Missing validation / NULL checks. fill_in_one_cache() dereferences *size and *line_size without NULL checks (pre-existing, but adjacent to the new code). assoc is checked, which is good, but there’s no power-of-two validation, and when assoc == 0 the leaf still publishes a size with number_of_sets = 0, which lscpu may render oddly.
12) “ecache” naming is platform-dependent. Historically “ecache” meant L2 on older SPARC; on S7/T7 the external/shared cache is L3. Using ecache_* for both with a hardcoded level 2 is semantically confusing; the level should come from MDESC.
13) Style / linkage. The new functions should probably be static; there’s mixed indentation versus the file’s tabs; and a checkpatch.pl --strict run would be worthwhile.
14) Error paths always return 0. init_cache_level() and populate_cache_leaves() publish leaves even if all MDESC cache data is zero/absent, yielding zero-size cache leaves rather than “unknown.”
15) leaf->priv = cs is probably harmless, but worth confirming nothing in the generic path or arch code expects a particular priv type.
A topology-driven fix — dynamic leaves from MDESC, correct levels, separate L2/L3 storage, and a shared_cpu_map keyed on sock_id/cluster — would make T2/T5240 report 8 MiB L2 in 2 instances and S7-2/T7-1 report their shared L3 correctly instead of 64× inflated.
One clarification on the T5240 lscpu output, since it can look partly correct: the L1d: 1 MiB (128 instances) and L1i: 2 MiB (128 instances) lines are actually right — L1 really is private per core on T2, so 128 instances is correct. The L2 line is the giveaway: it reports 512 MiB (128 instances), i.e. 128 × 4 MB, when a dual-socket T5240 has only 2 shared 4 MB L2 caches (one per socket), for 8 MiB total in 2 instances . The default “every leaf is private” assumption happens to be true for L1 and false for L2, which is why only the L2 line is inflated. This patch, as written, would leave that L2 line unchanged.
In short: on T2-class hardware (T5120/T5240) this patch produces the same incorrect lscpu output as unpatched mainline — it does not resolve the reported over-reporting there either.
Regards,
Tony