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

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Message-ID: <CAMj1kXF14uEM=eTsqKR8MBWYS_yebMQF5kca86gGyuXkjs+GRg@mail.gmail.com>
Date:   Sun, 20 Feb 2022 14:30:24 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Barry Song <21cnbao@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linuxarm <linuxarm@...wei.com>,
        Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH] irqchip/gic-v3: use dsb(ishst) to synchronize data to smp
 before issuing ipi

On Sat, 19 Feb 2022 at 10:57, Marc Zyngier <maz@...nel.org> wrote:
>
> On 2022-02-18 21:55, 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@...ilicon.com>
> > ---
> >  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);
> >
> >       for_each_cpu(cpu, mask) {
> >               u64 cluster_id = MPIDR_TO_SGI_CLUSTER_ID(cpu_logical_map(cpu));
>
> I'm not opposed to that change, but I'm pretty curious whether this
> makes
> any visible difference in practice. Could you measure the effect of this
> change
> for any sort of IPI heavy workload?
>

Does this have to be a DSB ?