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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Mar 1, 2016, at 1:25 PM, H. Peter Anvin hpa@zytor.com wrote:

> On 02/27/16 16:39, Mathieu Desnoyers wrote:
>> 
>> Very good points! Would the following interfaces be acceptable ?
>> 
>> /* This structure needs to be aligned cache line size. */
>> struct thread_local_abi {
>>         int32_t cpu_id;                               /* Aligned on
>> 32-bit. */
>>         uint32_t rseq_seqnum;                 /* Aligned on 32-bit. */
>>         uint64_t rseq_post_commit_ip;   /* Aligned on 64-bit. */
>>         /* Add new fields at the end. */
>> } __attribute__((packed));
>> 
> 
> First of all, DO NOT use __attribute__((packed)).  First of all, it
> buggers up the alignment of the *entire structure* (the alignment of a
> packed structure defaults to 1, and gcc will assume the whole structure
> is misaligned, generating unaligned access instructions on architectures
> which need them.)
> 
> Sadly gcc doesn't currently have an __attribute__ to express "error out
> on padding" which is what you actually want here.

Good point.

> 
> You may, however, want to add an explicit alignment attribute to make
> sure it is cache line aligned.

Good idea, will do!

> 
> Second, as far as the 32/64 bit issue is concerned, you have to order
> the fields so you always access the LSB.  This is probably the best way
> to do it:
> 
> #ifdef __LP64__
> # define __FIELD_32_64(field,n)	uint64_t field;
> #elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> # define __FIELD_32_64(field,n) uint32_t field, _unused ## n;
> #else
> # define __FIELD_32_64(field,n) uint32_t _unused ## n, field;
> #endif
> 
> All these macros are intrinsic to gcc (and hopefully to gcc-compatible
> compilers) so there are no header file dependencies.

Thanks for the hint. I'll try it out.

Mathieu


> 
> 	-hpa

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