Re: [PATCH] iommu/mediatek: Move the tlb_sync into tlb_flush

[Robin Murphy]

On 02/10/2019 06:18, Tomasz Figa wrote:
> Hi Yong,
>
> On Mon, Sep 30, 2019 at 2:42 PM Yong Wu <yong.wu@xxxxxxxxxxxx> wrote:
> > The commit 4d689b619445 ("iommu/io-pgtable-arm-v7s: Convert to IOMMU API
> > TLB sync") help move the tlb_sync of unmap from v7s into the iommu
> > framework. It helps add a new function "mtk_iommu_iotlb_sync", But it
> > lacked the dom->pgtlock, then it will cause the variable
> > "tlb_flush_active" may be changed unexpectedly, we could see this warning
> > log randomly:
>
> Thanks for the patch! Please see my comments inline.
>
> > mtk-iommu 10205000.iommu: Partial TLB flush timed out, falling back to
> > full flush
> >
> > To fix this issue, we can add dom->pgtlock in the "mtk_iommu_iotlb_sync".
> > And when checking this issue, we find that __arm_v7s_unmap call
> > io_pgtable_tlb_add_flush consecutively when it is supersection/largepage,
> > this also is potential unsafe for us. There is no tlb flush queue in the
> > MediaTek M4U HW. The HW always expect the tlb_flush/tlb_sync one by one.
> > If v7s don't always gurarantee the sequence, Thus, In this patch I move
> > the tlb_sync into tlb_flush(also rename the function deleting "_nosync").
> > and we don't care if it is leaf, rearrange the callback functions. Also,
> > the tlb flush/sync was already finished in v7s, then iotlb_sync and
> > iotlb_sync_all is unnecessary.
>
> Performance-wise, we could do much better. Instead of synchronously
> syncing at the end of mtk_iommu_tlb_add_flush(), we could sync at the
> beginning, if there was any previous flush still pending. We would
> also have to keep the .iotlb_sync() callback, to take care of waiting
> for the last flush. That would allow better pipelining with CPU in
> cases like this:
>
> for (all pages in range) {
>     change page table();
>     flush();
> }
>
> "change page table()" could execute while the IOMMU is flushing the
> previous change.

FWIW, given that the underlying invalidation mechanism is range-based, 
this driver would be an ideal candidate for making use of the new 
iommu_gather mechanism. As a fix for stable, though, simply ensuring 
that add_flush syncs any pending invalidation before issuing a new one 
sounds like a good idea (and probably a simpler patch too).

[...]
> > @@ -574,8 +539,7 @@ static int mtk_iommu_of_xlate(struct device *dev, struct of_phandle_args *args)
> >          .detach_dev     = mtk_iommu_detach_device,
> >          .map            = mtk_iommu_map,
> >          .unmap          = mtk_iommu_unmap,
> > -       .flush_iotlb_all = mtk_iommu_flush_iotlb_all,
>
> Don't we still want .flush_iotlb_all()? I think it should be more
> efficient in some cases than doing a big number of single flushes.
> (That said, the previous implementation didn't do any flush at all. It
> just waited for previously queued flushes to happen. Was that
> expected?)

Commit 07fdef34d2be ("iommu/arm-smmu-v3: Implement flush_iotlb_all 
hook") has an explanation of what the deal was there - similarly, it's 
probably worth this driver implementing it properly as well now (but 
that's really a separate patch).

Robin.