Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on the slow path

[Ingo Molnar <mingo@kernel.org>]

* Brian Gerst <brgerst@gmail.com> wrote:

> > We could adjust it a bit and check whether we're in C land (by checking rsp 
> > for ts) and jump into the slow path if we aren't, but I'm not sure this is a 
> > huge win.  It does save some rodata space by avoiding duplicating the table.
> 
> The syscall table is huge.  545*8 bytes, over a full page. Duplicating it for 
> just a few different entries is wasteful.

Note that what matters more is cache footprint, not pure size: 1K of RAM overhead 
for something as fundamental as system calls is trivial cost.

So the questions to ask are along these lines:

 - what is the typical locality of access (do syscall numbers cluster in time and 
   space)

 - how frequently would the two tables be accessed (is one accessed less 
   frequently than the other?)

 - subsequently how does the effective cache footprint change with the 
   duplication?

it might still end up not being worth it - but it's not the RAM cost that is the 
main factor IMHO.

Thanks,

	Ingo