Re: [PATCH v2 08/10] iommu/arm-smmu-v3: Shrink the cdtab l1_desc array

[Will Deacon <will@kernel.org>]

On Mon, Jun 10, 2024 at 09:31:17PM -0300, Jason Gunthorpe wrote:
> The top of the 2 level CD table is (at most) 1024 entries big, and two
> high order allocations are required. One of __le64 which is programmed
> into the HW (8k) and one of struct arm_smmu_l1_ctx_desc which holds the
> CPU pointer (16k).
> 
> There are two copies of the l2ptr_dma, one is stored in the struct
> arm_smmu_l1_ctx_desc, and another is encoded in the __le64 for the HW to
> use. Instead of storing two copies just decode the value from the __le64.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 41 +++++++++------------
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h |  1 -
>  2 files changed, 17 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 6245e2558e6a6a..dd65e27aebafd4 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -1167,31 +1167,19 @@ static void arm_smmu_sync_cd(struct arm_smmu_master *master,
>  	arm_smmu_cmdq_batch_submit(smmu, &cmds);
>  }
>  
> -static int arm_smmu_alloc_cd_leaf_table(struct arm_smmu_device *smmu,
> -					struct arm_smmu_l1_ctx_desc *l1_desc)
> +static void arm_smmu_write_cd_l1_desc(__le64 *dst, dma_addr_t l2ptr_dma)
>  {
> -	size_t size = CTXDESC_L2_ENTRIES * (CTXDESC_CD_DWORDS << 3);
> -
> -	l1_desc->l2ptr = dma_alloc_coherent(smmu->dev, size,
> -					    &l1_desc->l2ptr_dma, GFP_KERNEL);
> -	if (!l1_desc->l2ptr) {
> -		dev_warn(smmu->dev,
> -			 "failed to allocate context descriptor table\n");
> -		return -ENOMEM;
> -	}
> -	return 0;
> -}
> -
> -static void arm_smmu_write_cd_l1_desc(__le64 *dst,
> -				      struct arm_smmu_l1_ctx_desc *l1_desc)
> -{
> -	u64 val = (l1_desc->l2ptr_dma & CTXDESC_L1_DESC_L2PTR_MASK) |
> -		  CTXDESC_L1_DESC_V;
> +	u64 val = (l2ptr_dma & CTXDESC_L1_DESC_L2PTR_MASK) | CTXDESC_L1_DESC_V;
>  
>  	/* The HW has 64 bit atomicity with stores to the L2 CD table */
>  	WRITE_ONCE(*dst, cpu_to_le64(val));
>  }
>  
> +static dma_addr_t arm_smmu_cd_l1_get_desc(const __le64 *src)
> +{
> +	return le64_to_cpu(src) & CTXDESC_L1_DESC_L2PTR_MASK;
> +}

I'm assuming this is supposed to be *src, in which case this could be
accessing non-cacheable memory if the SMMU isn't coherent. That's why we
shadow everything.

If the current code isn't causing you any problems, please can we leave
it as-is?

Will