linux-kernel - Re: [RFC PATCH v1 2/8] iommu/arm-smmu-v3: Perform invalidations over installed

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Date:   Mon, 21 Aug 2023 08:57:58 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Michael Shavit <mshavit@...gle.com>
Cc:     Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, nicolinc@...dia.com, tina.zhang@...el.com,
        jean-philippe@...aro.org
Subject: Re: [RFC PATCH v1 2/8] iommu/arm-smmu-v3: Perform invalidations over
 installed_smmus

On Mon, Aug 21, 2023 at 04:33:36PM +0800, Michael Shavit wrote:
> > Notice that arm_smmu_tlb_inv_range_asid() already duplicates
> > arm_smmu_tlb_inv_range_domain().
> >
> > IMHO I would split the ATC step out of arm_smmu_mm_invalidate_range(),
> > get rid of arm_smmu_tlb_inv_range_domain(), and have the mmu notifier
> > just do as it already does:
> >
> >         if (!(smmu_domain->smmu->features & ARM_SMMU_FEAT_BTM))
> >                 arm_smmu_tlb_inv_range_domain_no_atc(start, size, smmu_mn->cd->asid,
> >                                             PAGE_SIZE, false, smmu_domain);
> >         arm_smmu_atc_inv_domain(smmu_domain, start, size);
> >
> > And make arm_smmu_tlb_inv_range_domain() just call
> >    arm_smmu_tlb_inv_range_domain_no_atc();
> >    arm_smmu_atc_inv_domain();
> 
> That's a nice clean-up but doesn't really solve the problem faced by this patch.
>
> This patch series eliminates the smmu_domain->smmu handle, replacing
> it for a list of SMMUs. So SVA can no longer optimize the
> arm_smmu_tlb_inv_range_asid call away by checking whether the SMMU BTM
> feature is enabled since there's now a list of SMMUs with possibly
> heterogeneous support for the feature. 

You could also go in the direction of making a SVA BTM and SV non-BTM
domain type and then you know what to do immediately in the notifier.

> Since there's now a loop over a series of SMMUs inside
> arm_smmu_tlb_inv_range_asid, it makes sense to move the check into
> that loop. This technically works because only SVA is calling
> arm_smmu_tlb_inv_range_asid but can (IMO) risk introducing bugs in
> the future since it's not obvious from the function name.

Well, I would remove the duplication and add an argument if you intend
to share the function that loops

Jason