----- On Feb 29, 2016, at 5:35 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:
> On Sun, Feb 28, 2016 at 02:32:28PM +0000, Mathieu Desnoyers wrote:
>> 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.
>
> Per the just proposed interface; discoverability would end with:
>
> thread_local_abi_register(NULL, TLA_ENABLE_RSEQ, 0);
>
> failing. This would indicate your kernel does not support (or your glibc
> failed to register, depending on error code I suppose).
>
> Then your program can either fall back to full atomics or just bail.
I think it's important that user-space fast-paths can quickly
detect whether the feature is enabled without having to rely on
always reading a separate cache-line. I've put together an ABI
proposal that take into account the feedback received so far.
The main trick here is to use "-1" value in cpu_id and rseq_seqnum
to mean "the feature is inactive" so user-space can call the system
call to register the feature, and the value "-2" can be set by the
kernel when it knows the feature is not available. It does mean
that seqnum would wrap from MAX_INT to 0 in the kernel, skipping
negative values.
Please let me know if I missed anything.
#ifdef __LP64__
# define TLABI_FIELD_u32_u64(field) uint64_t field
#elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
# define TLABI_FIELD_u32_u64(field) uint32_t field, _padding ## field
#else
# define TLABI_FIELD_u32_u64(field) uint32_t _padding ## field, field
#endif
/*
* The thread-local ABI structure needs to be aligned at least on 32
* bytes multiples.
*/
#define TLABI_ALIGNMENT 32
struct thread_local_abi {
/*
* Thread-local ABI cpu_id field.
* Updated by the kernel, and read by user-space with
* single-copy atomicity semantics. Aligned on 32-bit.
* Values:
* >= 0: CPU number of running thread.
* -1 (initial value): means the cpu_id feature is inactive.
* -2: cpu_id feature is not available.
*/
int32_t cpu_id;
/*
* Thread-local ABI rseq_seqnum field.
* Updated by the kernel, and read by user-space with
* single-copy atomicity semantics. Aligned on 32-bit.
* Values:
* >= 0: current seqnum for this thread (feature is active).
* -1 (initial value): means the rseq feature is inactive.
* -2: rseq feature is not available.
*/
int32_t rseq_seqnum;
/*
* Thread-local ABI rseq_post_commit_ip field.
* Updated by user-space, and read by the kernel with
* single-copy atomicity semantics.
* Aligned on 64-bit.
*/
TLABI_FIELD_u32_u64(rseq_post_commit_ip);
/* Add new fields at the end. */
} __attribute__ ((aligned(TLABI_ALIGNMENT)));
enum thread_local_abi_feature {
TLA_FEATURE_NONE = 0,
TLA_FEATURE_CPU_ID = (1 << 0),
TLA_FEATURE_RSEQ = (1 << 1),
};
/*
* Thread local ABI system call.
*
* First call with (NULL, 0, 0), returns the size of the struct
* thread_local_abi expected by the kernel, or -1 on error.
*
* Second, allocate a memory area to hold the struct thread_local_abi,
* and call with (ptr, 0, 0). Returns 0 on success, or -1 on error.
*
* Third, enable specific features by passing a mask, e.g. call with
* (NULL, TLA_FEATURE_CPU_ID | TLA_FEATURE_RSEQ, 0).
* Returns 0 on success, -1 on error.
*
* Then the fields associated with the enabled features are managed by
* the kernel.
*/
ssize_t thread_local_abi(struct thread_local_abi *tlabi,
uint64_t feature_mask, int flags);
Thanks for your feedback!
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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