linux-kernel - Re: [PATCH v2 1/3] iommu/arm-smmu-v3: put off the execution of TLBI* to reduce lock confliction

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Message-ID: <20171018125849.GD4077@arm.com>
Date:   Wed, 18 Oct 2017 13:58:49 +0100
From:   Will Deacon <will.deacon@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Hanjun Guo <guohanjun@...wei.com>,
        Libin <huawei.libin@...wei.com>, Jinyue Li <lijinyue@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH v2 1/3] iommu/arm-smmu-v3: put off the execution of TLBI*
 to reduce lock confliction

Hi Thunder,

On Tue, Sep 12, 2017 at 09:00:36PM +0800, Zhen Lei wrote:
> Because all TLBI commands should be followed by a SYNC command, to make
> sure that it has been completely finished. So we can just add the TLBI
> commands into the queue, and put off the execution until meet SYNC or
> other commands. To prevent the followed SYNC command waiting for a long
> time because of too many commands have been delayed, restrict the max
> delayed number.
> 
> According to my test, I got the same performance data as I replaced writel
> with writel_relaxed in queue_inc_prod.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>  drivers/iommu/arm-smmu-v3.c | 42 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 37 insertions(+), 5 deletions(-)

If we want to go down the route of explicit command batching, I'd much
rather do it by implementing the iotlb_range_add callback in the driver,
and have a fixed-length array of batched ranges on the domain. We could
potentially toggle this function pointer based on the compatible string too,
if it shows only to benefit some systems.

Will