Arnd,
On Wed, Jul 2, 2008 at 12:23 PM, Arnd Bergmann <[EMAIL PROTECTED]> wrote:
> On Tuesday 01 July 2008, stephane eranian wrote:
>> It may be necessary to extend the system calls with new generic or
>> architecture specific parameters, and this
>> without simply adding new system calls.
>>
>> ...
>> Although both structures are currently identical, they will
>> differ as more functionalities are added so better
>> to create two versions from the start.
>>
>> Provisions for extensions are provided by the reserved field in
>> each structure.
>
> What is the strategy regarding interface changes here? Are you confident
> that any change can be done in a forward- and backward-compatible way?
> If not, how do you address possible interface version mismatches?
>
I see two possible strategies:
1- the obvious one: add new syscalls
2- all system calls taking structure parameters have reserved
fields. They can be used to support extensions.
I see 3 problems with reserved fields:
- They are supposed to be all zeroes. Let's say, a new
perfmon version uses one to support an extension.
What if value 0 is valid for this extension, and we
execute an old application which does not know about it
and passes 0 in the corresponding reserved field?
This does not systematically translate into a problem.
Worst case, it may generate events which the old
application does not know about.
- what if we run out of reserved fields? Then we have not
other choice but to add a new syscall
- large reserved fields increase the cost of syscalls because
they need to be copied-in
It would be simpler with a multiplexing syscall, you just add a new
command. Older command would still work and
keep their own data structures.
I am tempted to transform the interface to have a mix of the two
approaches. I could clearly see that load/unload
and start/stop could be implemented by a generic pfm_control(int fd,
int CMD, void *arg, size_t arg_sz) call. To go
further, we could also group the write syscalls : pfm_write_regs(int
fd, int type, void *arg, size_t arg_sz) with type
indicating either config or data registers.
What do you think?
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