linux-kernel - Re: [PATCH] iommu/io-pgtable-arm: Optimize partial walk flush for large scatter-gather list

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Message-ID: <8535b6c757a5584b495f135f4377053c@codeaurora.org>
Date:   Wed, 16 Jun 2021 12:28:03 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Krishna Reddy <vdumpa@...dia.com>, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH] iommu/io-pgtable-arm: Optimize partial walk flush for
 large scatter-gather list

Hi Robin,

On 2021-06-15 19:23, Robin Murphy wrote:
> On 2021-06-15 12:51, Sai Prakash Ranjan wrote:

<snip>...

>> Hi @Robin, from these discussions it seems they are not ok with the 
>> change
>> for all SoC vendor implementations and do not have any data on such 
>> impact.
>> As I mentioned above, on QCOM platforms we do have several 
>> optimizations in HW
>> for TLBIs and would like to make use of it and reduce the unmap 
>> latency.
>> What do you think, should this be made implementation specific?
> 
> Yes, it sounds like there's enough uncertainty for now that this needs
> to be an opt-in feature. However, I still think that non-strict mode
> could use it generically, since that's all about over-invalidating to
> save time on individual unmaps - and relatively non-deterministic -
> already.
> 
> So maybe we have a second set of iommu_flush_ops, or just a flag
> somewhere to control the tlb_flush_walk functions internally, and the
> choice can be made in the iommu_get_dma_strict() test, but also forced
> on all the time by your init_context hook. What do you reckon?
> 

Sounds good to me. Since you mentioned non-strict mode using it 
generically,
can't we just set tlb_flush_all() in io_pgtable_tlb_flush_walk() like 
below
based on quirk so that we don't need to add any check in 
iommu_get_dma_strict()
and just force the new flush_ops in init_context hook?

if (iop->cfg.quirks & IO_PGTABLE_QUIRK_NON_STRICT) {
         iop->cfg.tlb->tlb_flush_all(iop->cookie);
         return;
}

Thanks,
Sai

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