Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()

Zoltan Menyhart
2006-06-16T12:37:05Z

Just to make sure I understand it correctly...
Assuming I have allocated per CPU data (numa control, etc.) pointed at by:

void *per_cpu[MAXCPUS];

Assuming a per CPU variable has got an "offset" in each per CPU data area.
Accessing this variable can be done as follows:

err = vgetcpu(&my_cpu, ...);
if (err)
goto ....
pointer = (typeof pointer) (per_cpu[my_cpu] + offset);
// use "pointer"...

It is hundred times more long than "__get_per_cpu(var)++".

As we do not know when we can be moved to another CPU,
"vgetcpu()" has to be called again after a "reasonable short" time.

My idea is to map the current task structure at an arch. dependent
virtual address into the user space (obviously in RO).

#define current ((struct task_struct *) 0x...)

No more need to for "vgetcpu()" at all. The example above becomes:

pointer = (typeof pointer) (per_cpu[current->thread_info.cpu] + offset);
// use "pointer"...

As obtaining "pointer" does not cost much, it can be re-calculated at
each usage => no problem to know when to recheck it, there is less chance for
using the data of a neighbor.

Regards,

Zoltan Menyhart