Re: [PATCH] irqchip/gic-v3: use dsb(ishst) to synchronize data to smp before issuing ipi

[Marc Zyngier]

On 2022-02-20 15:04, Russell King (Oracle) wrote:
> On Sat, Feb 19, 2022 at 05:55:49AM +0800, Barry Song wrote:
> > dsb(ishst) should be enough here as we only need to guarantee the
> > visibility of data to other CPUs in smp inner domain before we
> > send the ipi.
> >
> > Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> > ---
> >  drivers/irqchip/irq-gic-v3.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/irqchip/irq-gic-v3.c 
> > b/drivers/irqchip/irq-gic-v3.c
> > index 5e935d97207d..0efe1a9a9f3b 100644
> > --- a/drivers/irqchip/irq-gic-v3.c
> > +++ b/drivers/irqchip/irq-gic-v3.c
> > @@ -1211,7 +1211,7 @@ static void gic_ipi_send_mask(struct irq_data 
> > *d, const struct cpumask *mask)
> >  	 * Ensure that stores to Normal memory are visible to the
> >  	 * other CPUs before issuing the IPI.
> >  	 */
> > -	wmb();
> > +	dsb(ishst);
>
> On ARM, wmb() is a dsb(st) followed by other operations which may
> include a sync operation at the L2 cache, and SoC specific barriers
> for the bus. Hopefully, nothing will break if the sledge hammer is
> replaced by the tack hammer.

The saving grace is that ARMv8 forbids (as per D4.4.11) these 
SW-visible,
non architected caches (something like PL310 simply isn't allowed). 
Given
that GICv3 requires ARMv8 the first place, we should be OK.

As for SoC-specific bus barriers, I don't know of any that would affect
an ARMv8 based SoC. But I'm always prepared to be badly surprised...

        M.
--
Jazz is not dead. It just smells funny...