Re: [PATCHv2 2/3] iommu/io-pgtable: Optimize partial walk flush for large scatter-gather list

On 2021-06-21 06:47, Sai Prakash Ranjan wrote:

Hi,

On 2021-06-19 03:39, Doug Anderson wrote:

quoted

Hi,

On Thu, Jun 17, 2021 at 7:51 PM Sai Prakash Ranjan
[off-list ref] wrote:

quoted

Currently for iommu_unmap() of large scatter-gather list with page size
elements, the majority of time is spent in flushing of partial walks in
__arm_lpae_unmap() which is a VA based TLB invalidation invalidating
page-by-page on iommus like arm-smmu-v2 (TLBIVA) which do not support
range based invalidations like on arm-smmu-v3.2.

For example: to unmap a 32MB scatter-gather list with page size elements
(8192 entries), there are 16->2MB buffer unmaps based on the pgsize (2MB
for 4K granule) and each of 2MB will further result in 512 TLBIVAs 
(2MB/4K)
resulting in a total of 8192 TLBIVAs (512*16) for 16->2MB causing a huge
overhead.

So instead use tlb_flush_all() callback (TLBIALL/TLBIASID) to invalidate
the entire context for partial walk flush on select few platforms where
cost of over-invalidation is less than unmap latency

It would probably be worth punching this description up a little bit.
Elsewhere you said in more detail why this over-invalidation is less
of a big deal for the Qualcomm SMMU. It's probably worth saying
something like that here, too. Like this bit paraphrased from your
other email:

On qcom impl, we have several performance improvements for TLB cache
invalidations in HW like wait-for-safe (for realtime clients such as
camera and display) and few others to allow for cache lookups/updates
when TLBI is in progress for the same context bank.

Sure will add this info as well in the next version.

quoted

quoted

using the newly
introduced quirk IO_PGTABLE_QUIRK_TLB_INV_ALL. We also do this for
non-strict mode given its all about over-invalidation saving time on
individual unmaps and non-deterministic generally.

As per usual I'm mostly clueless, but I don't quite understand why you
want this new behavior for non-strict mode. To me it almost seems like
the opposite? Specifically, non-strict mode is already outside the
critical path today and so there's no need to optimize it. I'm
probably not explaining myself clearly, but I guess i'm thinking:

a) today for strict, unmap is in the critical path and it's important
to get it out of there. Getting it out of the critical path is so
important that we're willing to over-invalidate to speed up the
critical path.

b) today for non-strict, unmap is not in the critical path.

So I would almost expect your patch to _disable_ your new feature for
non-strict mappings, not auto-enable your new feature for non-strict
mappings.

If I'm babbling, feel free to ignore. ;-) Looking back, I guess Robin
was the one that suggested the behavior you're implementing, so it's
more likely he's right than I am. ;-)

Thanks for taking a look. Non-strict mode is only for leaf entries and
dma domains and this optimization is for non-leaf entries and is applicable
for both, see __arm_lpae_unmap(). In other words, if you have 
iommu.strict=0
(non-strict mode) and try unmapping a large sg buffer as the problem 
described
in the commit text, you would still go via this path in unmap and see the
delay without this patch. So what Robin suggested is that, let's do this
unconditionally for all users with non-strict mode as opposed to only
restricting it to implementation specific in case of strict mode.

Right, unmapping tables works out as a bit of a compromise for 
non-strict mode - we don't use a freelist to defer the freeing of 
pagetable pages, so we rely on making non-leaf invalidations 
synchronously to knock out walk caches which may be pointing to the page 
beforte we free it. We might actually be able to get away without that 
for non-strict unmaps, since partial walks pointing at freed memory 
probably aren't too much more hazardous than the equivalent leaf TLB 
entries while the IOVA region is held in the flush queue, but it 
certainly does matter for maps when we're knocking out a (presumably 
empty) table entry to put down a new block whose IOVA will be 
immediately live.

Robin.

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