linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Increase SMMU CMD queue poll timeout

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Date:   Wed, 26 Apr 2017 14:50:04 +0530
From:   Sunil Kovvuri <sunil.kovvuri@...il.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Geetha sowjanya <gakula@...iumnetworks.com>,
        "Goutham, Sunil" <Sunil.Goutham@...ium.com>,
        Catalin Marinas <catalin.marinas@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, Geetha <gakula@...ium.com>,
        Robin Murphy <robin.murphy@....com>,
        LAKML <linux-arm-kernel@...ts.infradead.org>, jcm@...hat.com
Subject: Re: [PATCH] iommu/arm-smmu-v3: Increase SMMU CMD queue poll timeout

On Mon, Apr 24, 2017 at 10:35 PM, Will Deacon <will.deacon@....com> wrote:
> On Mon, Apr 24, 2017 at 10:26:53PM +0530, Sunil Kovvuri wrote:
>> On Mon, Apr 24, 2017 at 9:38 PM, Will Deacon <will.deacon@....com> wrote:
>> > On Mon, Apr 24, 2017 at 05:29:36PM +0530, Geetha sowjanya wrote:
>> >> From: Geetha <gakula@...ium.com>
>> >>
>> >> When large memory is being unmapped, huge no of tlb invalidation cmds are
>> >> submitted followed by a SYNC command. This sometimes hits CMD queue full and
>> >> poll on queue drain is being timedout throwing error message 'CMD_SYNC timeout'.
>> >>
>> >> Although there is no functional issue, error message confuses user. Hence increased
>> >> poll timeout to 500us
>> >
>> > Hmm, what are you doing to unmap that much? Is this VFIO teardown? Do you
>> > have 7c6d90e2bb1a ("iommu/io-pgtable-arm: Fix iova_to_phys for block
>> > entries") applied?
>>
>> Yes it's VFIO teardown and again yes the above fix is applied.
>> But i didn't get how above fix is related.
>> TLB invalidation commands are submitted at 'arm_smmu_tlb_inv_range_nosync()'
>> and it's a loop over granule size.
>>
>> 1357         do {
>> 1358                 arm_smmu_cmdq_issue_cmd(smmu, &cmd);
>> 1359                 cmd.tlbi.addr += granule;
>> 1360         } while (size -= granule);
>>
>> So if invalidation size is big then huge no of invalidation commands
>> will be submitted
>> irrespective of fix that you pointed above, right ?
>
> VFIO has some logic to batch up invalidations, but this didn't work properly
> for us without the fix above. However, I guess you have a huge memory range
> that's mapped with 2M sections or something, so there are still loads of
> entries to invalidate.
>
> I would much prefer it if VFIO could just teardown the whole address space
> so that we could do an invalidate all, but there's a chicken-and-egg problem
> with page accounting iirc.
>

We can definitely look into this from VFIO perspective but for now I am guessing
this patch is fine, as no functionality is being changed.
What do you say ?

Thanks,
Sunil.