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

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Date:   Sun, 20 Feb 2022 15:21:15 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Barry Song <21cnbao@...il.com>, tglx@...utronix.de,
        will@...nel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, 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 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@...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);
> 
> 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...