Re: [RFC PATCH] thread_local_abi system call: caching current CPU number (x86)

On Fri, Jul 17, 2015 at 10:49:19AM +0000, Ben Maurer wrote:

Mathieu Desnoyers wrote:

quoted

Expose a new system call allowing threads to register a userspace memory
area where to store the current CPU number. Scheduler migration sets the

I really like that this approach makes it easier to add a per-thread interaction between userspace and the kernel in the future.

quoted

+       if (!tlap || t->thread_local_abi_len <
+                       offsetof(struct thread_local_abi, cpu)
+                       + sizeof(tlap->cpu))

Could you save a branch here by enforcing that thread_local_abi_len = 0 if thread_local_abi = null?

"saving a branch" doesn't seem like a good reason to do that; however,
it *is* the convention across other calls: if you pass 0, the pointer
is ignored, but if you pass non-zero, the pointer must be valid or you
get -EFAULT (or an actual segfault).

- Josh Triplett