[v1, 1/6] riscv: sched: defer restoring Vector context for user

[Andy Chiu]

On Mon, Jul 17, 2023 at 5:47 PM Conor Dooley <conor.dooley at microchip.com> wrote:
>
> Hey Andy,
> Small bit of minor nitpickery..
>
> On Sat, Jul 15, 2023 at 03:00:27PM +0000, Andy Chiu wrote:
> > User's will use its Vector registers only after the kernel really
>
> Looks like the ' here can be removed.
>
> > returns to the userspace. So we can delay restoring Vector registers as
> > long as we are still running in kernel mode. So, add a thread flag to
> > indicates the need of restoring Vector and do the restore at the last
> > arch-specific exit-to-user hook. This save the context restoring cost
> > when we switch over multiple processes that run V in kernel mode. For
> > example, if the kernel performs context swicth from A->B->C, and returns
>
> "a context switch"
>
> > to C's userspace, then there is no need for restoring B's V-register.
>
> "to restore"

Sorry for the poor english. Let me fix it in the next spin.

>
> >
> > Besides, this also prevents us from repeatedly restoring V context when
> > executing kernel-mode Vector multiple times for the upcoming kenel-mode
> > Vector patches.
> >
> > Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
> > ---
>
> > diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
> > index 97e6f65ec176..d83975efe866 100644
> > --- a/arch/riscv/include/asm/thread_info.h
> > +++ b/arch/riscv/include/asm/thread_info.h
> > @@ -101,12 +101,14 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
> >  #define TIF_NOTIFY_SIGNAL    9       /* signal notifications exist */
> >  #define TIF_UPROBE           10      /* uprobe breakpoint or singlestep */
> >  #define TIF_32BIT            11      /* compat-mode 32bit process */
> > +#define TIF_RISCV_V_DEFER_RESTORE    12
>
> The rest of these have a comment, should the new addition?

Yes, it should. How about this "defer restoring process's V-context"

>
> Anyway, no meaningful comments from me here Andy,
> Acked-by: Conor Dooley <conor.dooley at microchip.com>
>
> Thanks,
> Conor.