linux-kernel - Re: [PATCH v5 4/7] iommu: Factor iommu_iotlb_gather_is

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Message-ID: <618969ce-c220-a1bd-1e0e-33a22338e718@arm.com>
Date:   Tue, 13 Jul 2021 19:25:18 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nadav Amit <nadav.amit@...il.com>, Joerg Roedel <joro@...tes.org>
Cc:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, Nadav Amit <namit@...are.com>,
        Jiajun Cao <caojiajun@...are.com>
Subject: Re: [PATCH v5 4/7] iommu: Factor iommu_iotlb_gather_is_disjoint() out

On 2021-07-13 10:41, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
> 
> Refactor iommu_iotlb_gather_add_page() and factor out the logic that
> detects whether IOTLB gather range and a new range are disjoint. To be
> used by the next patch that implements different gathering logic for
> AMD.
> 
> Note that updating gather->pgsize unconditionally does not affect
> correctness as the function had (and has) an invariant, in which
> gather->pgsize always represents the flushing granularity of its range.
> Arguably, “size" should never be zero, but lets assume for the matter of
> discussion that it might.
> 
> If "size" equals to "gather->pgsize", then the assignment in question
> has no impact.
> 
> Otherwise, if "size" is non-zero, then iommu_iotlb_sync() would
> initialize the size and range (see iommu_iotlb_gather_init()), and the
> invariant is kept.
> 
> Otherwise, "size" is zero, and "gather" already holds a range, so
> gather->pgsize is non-zero and (gather->pgsize && gather->pgsize !=
> size) is true. Therefore, again, iommu_iotlb_sync() would be called and
> initialize the size.

With the caveat of one comment on the next patch...

Reviewed-by: Robin Murphy <robin.murphy@....com>

> Cc: Joerg Roedel <joro@...tes.org>
> Cc: Jiajun Cao <caojiajun@...are.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Lu Baolu <baolu.lu@...ux.intel.com>
> Cc: iommu@...ts.linux-foundation.org
> Cc: linux-kernel@...r.kernel.org>
> Acked-by: Will Deacon <will@...nel.org>
> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
>   include/linux/iommu.h | 34 ++++++++++++++++++++++++++--------
>   1 file changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index e554871db46f..979a5ceeea55 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -497,6 +497,28 @@ static inline void iommu_iotlb_sync(struct iommu_domain *domain,
>   	iommu_iotlb_gather_init(iotlb_gather);
>   }
>   
> +/**
> + * iommu_iotlb_gather_is_disjoint - Checks whether a new range is disjoint
> + *
> + * @gather: TLB gather data
> + * @iova: start of page to invalidate
> + * @size: size of page to invalidate
> + *
> + * Helper for IOMMU drivers to check whether a new range and the gathered range
> + * are disjoint. For many IOMMUs, flushing the IOMMU in this case is better
> + * than merging the two, which might lead to unnecessary invalidations.
> + */
> +static inline
> +bool iommu_iotlb_gather_is_disjoint(struct iommu_iotlb_gather *gather,
> +				    unsigned long iova, size_t size)
> +{
> +	unsigned long start = iova, end = start + size - 1;
> +
> +	return gather->end != 0 &&
> +		(end + 1 < gather->start || start > gather->end + 1);
> +}
> +
> +
>   /**
>    * iommu_iotlb_gather_add_range - Gather for address-based TLB invalidation
>    * @gather: TLB gather data
> @@ -533,20 +555,16 @@ static inline void iommu_iotlb_gather_add_page(struct iommu_domain *domain,
>   					       struct iommu_iotlb_gather *gather,
>   					       unsigned long iova, size_t size)
>   {
> -	unsigned long start = iova, end = start + size - 1;
> -
>   	/*
>   	 * If the new page is disjoint from the current range or is mapped at
>   	 * a different granularity, then sync the TLB so that the gather
>   	 * structure can be rewritten.
>   	 */
> -	if (gather->pgsize != size ||
> -	    end + 1 < gather->start || start > gather->end + 1) {
> -		if (gather->pgsize)
> -			iommu_iotlb_sync(domain, gather);
> -		gather->pgsize = size;
> -	}
> +	if ((gather->pgsize && gather->pgsize != size) ||
> +	    iommu_iotlb_gather_is_disjoint(gather, iova, size))
> +		iommu_iotlb_sync(domain, gather);
>   
> +	gather->pgsize = size;
>   	iommu_iotlb_gather_add_range(gather, iova, size);
>   }
>   
>