Re: [RFC PATCH] iommu: Optimize IOMMU UnMap

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

On 23/05/2024 4:19 am, Ashish Mhetre wrote:
> The current __arm_lpae_unmap() function calls dma_sync() on individual
> PTEs after clearing them. By updating the __arm_lpae_unmap() to call
> dma_sync() once for all cleared PTEs, the overall performance can be
> improved 25% for large buffer sizes.
> Below is detailed analysis of average unmap latency(in us) with and
> without this optimization obtained by running dma_map_benchmark for
> different buffer sizes.
> 
> Size	Time W/O	Time With	% Improvement
> 	Optimization	Optimization
> 	(us)		(us)
> 
> 4KB	3.0		3.1		-3.33
> 1MB	250.3		187.9		24.93

This seems highly suspect - the smallest possible block size is 2MB so a 
1MB unmap should not be affected by this path at all.

> 2MB	493.7		368.7		25.32
> 4MB	974.7		723.4		25.78

I'm guessing this is on Tegra with the workaround to force everything to 
PAGE_SIZE? In the normal case a 2MB unmap should be nominally *faster* 
than 4KB, since it would also be a single PTE, but with one fewer level 
of table to walk to reach it. The 25% figure is rather misleading if 
it's only a mitigation of an existing erratum workaround, and the actual 
impact on the majority of non-broken systems is unmeasured.

(As an aside, I think that workaround itself is a bit broken, since at 
least on Tegra234 with Cortex-A78, PAGE_SIZE could be 16KB which MMU-500 
doesn't support.)

> Signed-off-by: Ashish Mhetre <amhetre@nvidia.com>
> ---
>   drivers/iommu/io-pgtable-arm.c | 34 +++++++++++++++++++++++++---------
>   1 file changed, 25 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> index 3d23b924cec1..94094b711cba 100644
> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -256,13 +256,15 @@ static void __arm_lpae_sync_pte(arm_lpae_iopte *ptep, int num_entries,
>   				   sizeof(*ptep) * num_entries, DMA_TO_DEVICE);
>   }
>   
> -static void __arm_lpae_clear_pte(arm_lpae_iopte *ptep, struct io_pgtable_cfg *cfg)
> +static void __arm_lpae_clear_pte(arm_lpae_iopte *ptep, struct io_pgtable_cfg *cfg, int num_entries)
>   {
> +	int i;
>   
> -	*ptep = 0;
> +	for (i = 0; i < num_entries; i++)
> +		ptep[i] = 0;
>   
>   	if (!cfg->coherent_walk)
> -		__arm_lpae_sync_pte(ptep, 1, cfg);
> +		__arm_lpae_sync_pte(ptep, num_entries, cfg);
>   }
>   
>   static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
> @@ -633,13 +635,25 @@ static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
>   	if (size == ARM_LPAE_BLOCK_SIZE(lvl, data)) {
>   		max_entries = ARM_LPAE_PTES_PER_TABLE(data) - unmap_idx_start;
>   		num_entries = min_t(int, pgcount, max_entries);
> -
> -		while (i < num_entries) {
> -			pte = READ_ONCE(*ptep);
> +		arm_lpae_iopte *pte_flush;
> +		int j = 0;
> +
> +		pte_flush = kvcalloc(num_entries, sizeof(*pte_flush), GFP_ATOMIC);

kvmalloc() with GFP_ATOMIC isn't valid. However, I'm not sure if there 
isn't a more fundamental problem here - Rob, Boris; was it just the map 
path, or would any allocation on unmap risk the GPU reclaim deadlock 
thing as well?

Thanks,
Robin.

> +		if (pte_flush) {
> +			for (j = 0; j < num_entries; j++) {
> +				pte_flush[j] = READ_ONCE(ptep[j]);
> +				if (WARN_ON(!pte_flush[j]))
> +					break;
> +			}
> +			__arm_lpae_clear_pte(ptep, &iop->cfg, j);
> +		}
> +		while (i < (pte_flush ? j : num_entries)) {
> +			pte = pte_flush ? pte_flush[i] : READ_ONCE(*ptep);
>   			if (WARN_ON(!pte))
>   				break;
>   
> -			__arm_lpae_clear_pte(ptep, &iop->cfg);
> +			if (!pte_flush)
> +				__arm_lpae_clear_pte(ptep, &iop->cfg, 1);
>   
>   			if (!iopte_leaf(pte, lvl, iop->fmt)) {
>   				/* Also flush any partial walks */
> @@ -649,10 +663,12 @@ static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
>   			} else if (!iommu_iotlb_gather_queued(gather)) {
>   				io_pgtable_tlb_add_page(iop, gather, iova + i * size, size);
>   			}
> -
> -			ptep++;
> +			if (!pte_flush)
> +				ptep++;
>   			i++;
>   		}
> +		if (pte_flush)
> +			kvfree(pte_flush);
>   
>   		return i * size;
>   	} else if (iopte_leaf(pte, lvl, iop->fmt)) {