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

[Robin Murphy <robin.murphy@arm.com>]

On 2021-06-10 06:24, Sai Prakash Ranjan wrote:
> Hi Robin,
> 
> On 2021-06-10 00:14, Robin Murphy wrote:
>> On 2021-06-09 15:53, Sai Prakash Ranjan wrote:
>>> 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 (TLBIVA for
>>> arm-smmu).
>>>
>>> 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 io_pgtable_tlb_flush_all() to invalidate the entire 
>>> context
>>> if size (pgsize) is greater than the granule size (4K, 16K, 64K). For 
>>> this
>>> example of 32MB scatter-gather list unmap, this results in just 16 ASID
>>> based TLB invalidations or tlb_flush_all() callback (TLBIASID in case of
>>> arm-smmu) as opposed to 8192 TLBIVAs thereby increasing the 
>>> performance of
>>> unmaps drastically.
>>>
>>> Condition (size > granule size) is chosen for io_pgtable_tlb_flush_all()
>>> because for any granule with supported pgsizes, we will have at least 
>>> 512
>>> TLB invalidations for which tlb_flush_all() is already recommended. For
>>> example, take 4K granule with 2MB pgsize, this will result in 512 TLBIVA
>>> in partial walk flush.
>>>
>>> Test on QTI SM8150 SoC for 10 iterations of iommu_{map_sg}/unmap:
>>> (average over 10 iterations)
>>>
>>> Before this optimization:
>>>
>>>      size        iommu_map_sg      iommu_unmap
>>>        4K            2.067 us         1.854 us
>>>       64K            9.598 us         8.802 us
>>>        1M          148.890 us       130.718 us
>>>        2M          305.864 us        67.291 us
>>>       12M         1793.604 us       390.838 us
>>>       16M         2386.848 us       518.187 us
>>>       24M         3563.296 us       775.989 us
>>>       32M         4747.171 us      1033.364 us
>>>
>>> After this optimization:
>>>
>>>      size        iommu_map_sg      iommu_unmap
>>>        4K            1.723 us         1.765 us
>>>       64K            9.880 us         8.869 us
>>>        1M          155.364 us       135.223 us
>>>        2M          303.906 us         5.385 us
>>>       12M         1786.557 us        21.250 us
>>>       16M         2391.890 us        27.437 us
>>>       24M         3570.895 us        39.937 us
>>>       32M         4755.234 us        51.797 us
>>>
>>> This is further reduced once the map/unmap_pages() support gets in which
>>> will result in just 1 tlb_flush_all() as opposed to 16 tlb_flush_all().
>>>
>>> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
>>> ---
>>>   drivers/iommu/io-pgtable-arm.c | 7 +++++--
>>>   1 file changed, 5 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/iommu/io-pgtable-arm.c 
>>> b/drivers/iommu/io-pgtable-arm.c
>>> index 87def58e79b5..c3cb9add3179 100644
>>> --- a/drivers/iommu/io-pgtable-arm.c
>>> +++ b/drivers/iommu/io-pgtable-arm.c
>>> @@ -589,8 +589,11 @@ static size_t __arm_lpae_unmap(struct 
>>> arm_lpae_io_pgtable *data,
>>>             if (!iopte_leaf(pte, lvl, iop->fmt)) {
>>>               /* Also flush any partial walks */
>>> -            io_pgtable_tlb_flush_walk(iop, iova, size,
>>> -                          ARM_LPAE_GRANULE(data));
>>> +            if (size > ARM_LPAE_GRANULE(data))
>>> +                io_pgtable_tlb_flush_all(iop);
>>> +            else
>>
>> Erm, when will the above condition ever not be true? ;)
>>
> 
> Ah right, silly me :)
> 
>> Taking a step back, though, what about the impact to drivers other
>> than SMMUv2?
> 
> Other drivers would be msm_iommu.c, qcom_iommu.c which does the same
> thing as arm-smmu-v2 (page based invalidations), then there is ipmmu-vmsa.c
> which does tlb_flush_all() for flush walk.
> 
>> In particular I'm thinking of SMMUv3.2 where the whole
>> range can be invalidated by VA in a single command anyway, so the
>> additional penalties of TLBIALL are undesirable.
>>
> 
> Right, so I am thinking we can have a new generic quirk 
> IO_PGTABLE_QUIRK_RANGE_INV
> to choose between range based invalidations(tlb_flush_walk) and 
> tlb_flush_all().
> In this case of arm-smmu-v3.2, we can tie up ARM_SMMU_FEAT_RANGE_INV 
> with this quirk
> and have something like below, thoughts?
> 
> if (iop->cfg.quirks & IO_PGTABLE_QUIRK_RANGE_INV)
>          io_pgtable_tlb_flush_walk(iop, iova, size,
>                                    ARM_LPAE_GRANULE(data));
> else
>          io_pgtable_tlb_flush_all(iop);

The design here has always been that io-pgtable says *what* needs 
invalidating, and we left it up to the drivers to decide exactly *how*. 
Even though things have evolved a bit I don't think that has 
fundamentally changed - tlb_flush_walk is now only used in this one 
place (technically I suppose it could be renamed tlb_flush_table but 
it's not worth the churn), so drivers can implement their own preferred 
table-invalidating behaviour even more easily than choosing whether to 
bounce a quirk through the common code or not. Consider what you've 
already seen for the Renesas IPMMU, or SMMUv1 stage 2...

I'm instinctively a little twitchy about making this a blanket 
optimisation for SMMUv2 since I still remember the palaver with our 
display and MMU-500 integrations, where it had to implement the dodgy 
"prefetch" register to trigger translations before scanning out a frame 
since it couldn't ever afford a TLB miss, thus TLBIALL when freeing an 
old buffer would be a dangerous hammer to swing. However IIRC it also 
had to ensure everything was mapped as 2MB blocks to guarantee fitting 
everything in the TLBs in the first place, so I guess it would still 
work out OK due to never realistically unmapping a whole table at once 
anyway.

Cheers,
Robin.