On Mon, Dec 4, 2023 at 3:37 PM Luc Van Oostenryck
<luc.vanoostenryck@xxxxxxxxx> wrote:
>
> On Mon, Dec 04, 2023 at 02:57:53PM +0100, Uros Bizjak wrote:
> > On Mon, Dec 4, 2023 at 2:05 PM Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx> wrote:
> >
> > __seg_gs qualified variables can be referenced anywhere in the code, e.g.:
> >
> > --cut here--
> > __seg_gs int m;
> >
> > int foo (void)
> > {
> > return m + m;
> > }
>
> Yes, of course. But my point was not if they *can* be dereferenced but rather
> if *in the kernel, do want or not to use 'naked' __seg_gs qualified variable*.
> In other words, do we want to be warned if someone, somewhere try to
> dereference such qualified variable without using the accessor function?
> And I think the answer to this question should be 'yes'.
Not really - when used as e.g.:
extern const struct pcpu_hot __seg_gs const_pcpu_hot;
we can use variable directly, e.g.:
return const_pcpu_hot.current_task;
This way, the compiler is able to significantly reduce the number of
loads from current_task, which is not achievable when the accessor is
used to dereference the structure. In the x86/percpu branch, the
number of loads from current_task were reduced from ~4700 to 3201 by
using 'naked' __seg_gs qualified variable. IOW, the benefit of using
__seg_gs is to be able to access the naked variable.
Uros.
