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

[Mathieu Desnoyers]

----- On Feb 27, 2016, at 1:35 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx wrote:

> On Sat, Feb 27, 2016 at 6:58 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>
>> Paul's patches have the following structure:
>>
>> struct thread_local_abi {
>>         union {
>>                 struct {
>>                         u32     cpu_id;
>>                         u32     seq;
>>                 };
>>                 u64 cpu_seq;
>>         };
>>         unsigned long post_commit_ip;
>> };
> 
> Please don't do "unsigned long" in ABI structures any more.
> 
> Make it u64, and make sure it is 64-bit aligned (which it would be in
> this case). Make it so that we don't have to have separate compat
> paths.

AFAIU, this "post_commit_ip" field is expected to be updated
with a single-copy-store by user-space. If we want to handle both
32-bit and 64-bit processes, how do you recommend doing this
without an unsigned long type ?

A 64-bit integer would not be a single-copy store for
32-bit processes, but a 32-bit integer would not be large
enough for 64-bit processes.

Would a

union {
    uint32_t val32;
    uint64_t val64;
} field;

be an acceptable option ? Then the kernel could use
one field or the other depending on the process bitness.

Thanks,

Mathieu

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