On Fri, Jan 18, 2019 at 05:30:02PM +0000, Catalin Marinas wrote:
> On Fri, Jan 18, 2019 at 04:57:32PM +0000, Julien Thierry wrote:
> > On 18/01/2019 16:09, Catalin Marinas wrote:
> > > On Tue, Jan 08, 2019 at 02:07:30PM +0000, Julien Thierry wrote:
> > >> + asm volatile(ALTERNATIVE(
> > >> + "nop",
> > >> + "mrs_s %0, " __stringify(SYS_ICC_PMR_EL1),
> > >> + ARM64_HAS_IRQ_PRIO_MASKING)
> > >> + : "=&r" (pmr)
> > >> :
> > >> : "memory");
> > >> +
> > >> + return _get_irqflags(daif_bits, pmr);
> > >> +}
> > >
> > > I find this confusing spread over two inline asm statements. IIUC, you
> > > want something like below (it could be written as inline asm but I need
> > > to understand it first):
> > >
> > > daif_bits = read_sysreg(daif);
> > >
> > > if (system_uses_irq_prio_masking()) {
> > > pmr = read_gicreg(ICC_PMR_EL1);
> > > flags = pmr & ~(daif_bits & PSR_I_BIT);
> > > } else {
> > > flags = daif_bits;
> > > }
> > >
> > > return flags;
> > >
> > > In the case where the interrupts are disabled at the PSR level, is the
> > > PMR value still relevant? Could we just return the GIC_PRIO_IRQOFF?
> > > Something like:
> > >
> > > flags = read_sysreg(daif);
> > >
> > > if (system_uses_irq_prio_masking())
> > > flags = flags & PSR_I_BIT ?
> > > GIC_PRIO_IRQOFF : read_gicreg(ICC_PMR_EL1);
> > >
> >
> > You're right, returning GIC_PRIO_IRQOFF should be good enough (it is
> > actually what happens in this version because GIC_PRIO_IRQOFF ==
> > GIC_PRIO_IRQON & ~PSR_I_BIT happens to be true).
>
> This wasn't entirely clear to me, I got confused by:
>
> + BUILD_BUG_ON(GIC_PRIO_IRQOFF < (GIC_PRIO_IRQON & ~PSR_I_BIT)); \
>
> and I thought there isn't necessarily an equality between the two.
>
> > Your suggestion would
> > make things easier to reason about. Maybe something like:
> >
> >
> > static inline unsigned long arch_local_save_flags(void)
> > {
> > unsigned long daif_bits;
> > unsigned long prio_off = GIC_PRIO_IRQOFF;
> >
> > daif_bits = read_sysreg(daif);
> >
> > asm volatile(ALTERNATIVE(
> > "mov %0, %1\n"
> > "nop\n"
> > "nop",
> > "mrs %0, SYS_ICC_PMR_EL1\n"
> > "ands %1, %1, PSR_I_BIT\n"
> > "csel %0, %0, %2, eq")
> > : "=&r" (flags)
> > : "r" (daif_bits), "r" (prio_off)
> > : "memory");
> >
> > return flags;
> > }
>
> It looks fine. If you turn the BUILD_BUG_ON into a !=, you could
> probably simplify the asm a bit (though the number of instructions
> generated would probably be the same). Untested:
>
> static inline unsigned long arch_local_save_flags(void)
> {
> unsigned long flags;
>
> flags = read_sysreg(daif);
>
> asm volatile(ALTERNATIVE(
> "nop",
> "bic %0, %1, %2")
> : "=&r" (flags)
> : "r" (flags & PSR_I_BIT), "r" (GIC_PRIO_IRQOFF)
> : "memory");
Ah, I missed a read from SYS_ICC_PMR_EL1 here. Anyway, the idea was that
you don't need to set prio_off to a variable, just pass "r" (constant)
here and the compiler does the trick.
--
Catalin
