On Mon, May 14, 2018 at 03:43:36PM +0100, Dave Martin wrote:
> On Mon, May 14, 2018 at 12:58:05PM +0100, Mark Rutland wrote:
> > On Mon, May 14, 2018 at 12:07:30PM +0100, Dave Martin wrote:
> > > On Mon, May 14, 2018 at 10:46:32AM +0100, Mark Rutland wrote:
> > > > +{
> > > > + if (!system_supports_sve())
> > > > + return;
> > > > +
> > > > + /*
> > > > + * task_fpsimd_load() won't be called to update CPACR_EL1 in
> > > > + * ret_to_user unless TIF_FOREIGN_FPSTATE is still set, which
> > > > only
> > > > + * happens if a context switch or kernel_neon_begin() or context
> > > > + * modification (sigreturn, ptrace) intervenes.
> > > > + * So, ensure that CPACR_EL1 is already correct for the
> > > > fast-path case.
> > > > + */
> > > > + if (test_and_clear_thread_flag(TIF_SVE))
> > > > + sve_user_disable();
> > >
> > > sve_user_disable() is already inline, and incorporates the if()
> > > internally via sysreg_clear_set().
> > >
> > > So, should this just be
> > >
> > > clear_thread_flag(TIF_SVE);
> > > sve_user_disable();
> >
> > Sure. That does mean we'll unconditionally read cpacr_el1, but I assume
> > you're happy with that. I'll note the difference in the commit message.
>
> This is what the code does today, conditioned no system_supports_sve().
>
> I'm assuming that reading CPACR_EL1 is cheap ... or have you come across
> counterexamples to that?
I have no data either way. :)
> > > > +}
> > > > +
> > > > +extern syscall_fn_t sys_call_table[];
> > > > +
> > > > +asmlinkage void el0_svc_handler(struct pt_regs *regs)
> > > > +{
> > >
> > > if (system_supports_sve()) ?
> > >
> > > > + sve_user_disable();
> > >
> > > Or should this be replaced by a call to sve_user_reset()?
> > >
> > > I suspect the latter, since we do want to be clearing TIF_SVE here too.
> >
> > Yes, this was mean to be sve_user_reset().
>
> OK. Just to be clear, I think there should be a system_supports_sve()
> check here (in case that wasn't obvious from my previous reply).
I understood that; the check is inside sve_user_reset(), which I had
mean to call here.
With your above comments, I now have the following:
static inline void sve_user_reset(void)
{
if (!system_supports_sve())
return;
/*
* task_fpsimd_load() won't be called to update CPACR_EL1 in
* ret_to_user unless TIF_FOREIGN_FPSTATE is still set, which only
* happens if a context switch or kernel_neon_begin() or context
* modification (sigreturn, ptrace) intervenes.
* So, ensure that CPACR_EL1 is already correct for the fast-path case.
*/
clear_thread_flag(TIF_SVE);
sve_user_disable();
}
asmlinkage void el0_svc_handler(struct pt_regs *regs)
{
sve_user_reset();
el0_svc_common(regs, regs->regs[8], __NR_syscalls, sys_call_table);
}
... which I think alleviates that concern?
Thanks,
Mark.
