On Mon, Jul 20, 2020 at 06:00:50PM +0100, Dave P Martin wrote:
> On Mon, Jul 20, 2020 at 04:30:35PM +0100, Kevin Brodsky wrote:
> > On 15/07/2020 18:08, Catalin Marinas wrote:
> > >+void mte_thread_switch(struct task_struct *next)
> > >+{
> > >+ if (!system_supports_mte())
> > >+ return;
> > >+
> > >+ /* avoid expensive SCTLR_EL1 accesses if no change */
> > >+ if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
> >
> > I think this could be improved by checking whether `next` is a kernel
> > thread, in which case thread.sctlr_tcf0 is 0 but there is no point in
> > setting SCTLR_EL1.TCF0, since there should not be any access via TTBR0.
>
> Out of interest, do we have a nice way of testing for a kernel thread
> now?
>
> I remember fpsimd_thread_switch() used to check for task->mm, but we
> seem to have got rid of that at some point. set_mm() can defeat this,
> and anyway the heavy lifting for FPSIMD is now deferred until returning
> to userspace.
I think a better way is (current->flags & PF_KTHREAD). kthread_use_mm()
indeed changes current->mm.
--
Catalin
