Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

----- On Feb 27, 2016, at 7:57 PM, Linus Torvalds torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org wrote:

On Sat, Feb 27, 2016 at 4:39 PM, Mathieu Desnoyers
[off-list ref] wrote:

quoted

I'm particularly interested to know what are the best practices to
deal with an extensible bitfield (the features mask). cpu_set_t
and sigmask each seem to do their own thing.

Quite frankly, why would the kernel ever touch anything else?

And if the kernel doesn't touch anything else, why make it part of the ABI?

I don't see why the kernel would ever want to have a more complex
interface. Explain.

The part of ABI I'm trying to express here is for discoverability
of available features by user-space. For instance, a kernel
could be configured with "CONFIG_RSEQ=n", and userspace should
not rely on the rseq fields of the thread-local ABI in that case.

The initial idea I had was to populate a mask of available features
(hence my question above), but now that I think about it, we could
perhaps have a "query" system call receiving a "feature number", no
mask needed then. E.g.:

enum thread_local_abi_features {
    THREAD_LOCAL_FEATURE_CPU_ID = 0,
    THREAD_LOCAL_FEATURE_RSEQ = 1,
    /* Add future features here. */
};

int thread_local_abi_feature(uint64_t feature);

Another option would be to rely on specific "uninitialized"
values for each feature in struct thread_local_abi (e.g. -1
for cpu_id). We may need to reserve extra space for
"feature enabled" booleans in cases where the uninitialized
value is also used when initialized (e.g. a sequence counteR).
The advantage of using the uninitialized value and/or the
"boolean" within the struct thread_local_abi is that testing
whether the feature is active can be done by reading from
the same cache-line as when using the feature (in user-space).

Not sure what would be the best option here.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com