[v6,1/4] bpf: add kfunc for populating cpumask bits

Commit Message

Emil Tsalapatis March 7, 2025, 3:38 p.m. UTC

Add a helper kfunc that sets the bitmap of a bpf_cpumask from BPF memory.

Signed-off-by: Emil Tsalapatis (Meta) <[e-mail]>
Acked-by: Hou Tao <[e-mail]>
---
 kernel/bpf/cpumask.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

Comments

Tejun Heo March 7, 2025, 3:50 p.m. UTC | #1

On Fri, Mar 07, 2025 at 10:38:44AM -0500, Emil Tsalapatis wrote:
> Add a helper kfunc that sets the bitmap of a bpf_cpumask from BPF memory.
> 
> Signed-off-by: Emil Tsalapatis (Meta) <[e-mail]>
> Acked-by: Hou Tao <[e-mail]>

Would a kfunc to transfer it in the other direction be useful too? If so,
how would that function be named?

Thanks.

Emil Tsalapatis March 7, 2025, 4:53 p.m. UTC | #2

On Fri, Mar 7, 2025 at 11:28 AM Tejun Heo <[e-mail]> wrote:
>
> On Fri, Mar 07, 2025 at 06:28:21AM -1000, Tejun Heo wrote:
> > Hello,
> >
> > On Fri, Mar 07, 2025 at 11:18:46AM -0500, Emil Tsalapatis wrote:
> > > On Fri, Mar 7, 2025 at 10:50 AM Tejun Heo <[e-mail]> wrote:
> > > >
> > > > On Fri, Mar 07, 2025 at 10:38:44AM -0500, Emil Tsalapatis wrote:
> > > > > Add a helper kfunc that sets the bitmap of a bpf_cpumask from BPF memory.
> > > > >
> > > > > Signed-off-by: Emil Tsalapatis (Meta) <[e-mail]>
> > > > > Acked-by: Hou Tao <[e-mail]>
> > > >
> > > > Would a kfunc to transfer it in the other direction be useful too? If so,
> > > > how would that function be named?
> > > >
> > >
> > > We could add one, but it is not necessary because the BPF program can do the
> > > copy itself by reading the struct cpumask e.g. like this:
> > >
> > > https://github.com/sched-ext/scx/blob/ecdba1f4d9d518bd6a58343cd303187155a39bf3/scheds/rust/scx_wd40/src/bpf/cpumask.bpf.c#L184
> >
> > Ah, right.
> >
> > > If we added a function going bpf_cpumask -> BPF would
> > > bpf_cpumask_into() work as a name?
> >
> > Yeah, was mostly thinking whether the _populate() name would look weird if
> > we need something in the other direction. If we don't, the name is fine:
> >
> >  Acked-by: Tejun Heo <[e-mail]>
>
> Oops, you dropped the cc list. Can you restore the cc list and quote the
> whole exchange?
>

Ah sorry about that, I fat fingered the reply. Hopefully this properly
restores the thread in lore.kernel.org.

> Thanks.
>
> --
> tejun

Patch

diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
index cfa1c18e3a48..77900cbbbd75 100644
--- a/kernel/bpf/cpumask.c
+++ b/kernel/bpf/cpumask.c
@@ -420,6 +420,38 @@  __bpf_kfunc u32 bpf_cpumask_weight(const struct cpumask *cpumask)
 	return cpumask_weight(cpumask);
 }
 
+/**
+ * bpf_cpumask_populate() - Populate the CPU mask from the contents of
+ * a BPF memory region.
+ *
+ * @cpumask: The cpumask being populated.
+ * @src: The BPF memory holding the bit pattern.
+ * @src__sz: Length of the BPF memory region in bytes.
+ *
+ * Return:
+ * * 0 if the struct cpumask * instance was populated successfully.
+ * * -EACCES if the memory region is too small to populate the cpumask.
+ * * -EINVAL if the memory region is not aligned to the size of a long
+ *   and the architecture does not support efficient unaligned accesses.
+ */
+__bpf_kfunc int bpf_cpumask_populate(struct cpumask *cpumask, void *src, size_t src__sz)
+{
+	unsigned long source = (unsigned long)src;
+
+	/* The memory region must be large enough to populate the entire CPU mask. */
+	if (src__sz < bitmap_size(nr_cpu_ids))
+		return -EACCES;
+
+	/* If avoiding unaligned accesses, the input region must be aligned to the nearest long. */
+	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
+		!IS_ALIGNED(source, sizeof(long)))
+		return -EINVAL;
+
+	bitmap_copy(cpumask_bits(cpumask), src, nr_cpu_ids);
+
+	return 0;
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(cpumask_kfunc_btf_ids)
@@ -448,6 +480,7 @@  BTF_ID_FLAGS(func, bpf_cpumask_copy, KF_RCU)
 BTF_ID_FLAGS(func, bpf_cpumask_any_distribute, KF_RCU)
 BTF_ID_FLAGS(func, bpf_cpumask_any_and_distribute, KF_RCU)
 BTF_ID_FLAGS(func, bpf_cpumask_weight, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_populate, KF_RCU)
 BTF_KFUNCS_END(cpumask_kfunc_btf_ids)
 
 static const struct btf_kfunc_id_set cpumask_kfunc_set = {