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

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Message-ID: <YhJYu61HkL9Ywq0R@shell.armlinux.org.uk>
Date:   Sun, 20 Feb 2022 15:05:31 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Marc Zyngier <maz@...nel.org>, 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 Sun, Feb 20, 2022 at 02:30:24PM +0100, Ard Biesheuvel wrote:
> 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 ?

Are you suggesting that smp_wmb() may suffice (which is a dmb(ishst)) ?

-- 
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