linux-kernel - Re: [PATCH 1/2] iommu/virtio: Make use of ops->iotlb_sync

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Date:   Mon, 4 Sep 2023 16:26:32 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        virtualization@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] iommu/virtio: Make use of ops->iotlb_sync_map

Hi Niklas,

Thanks for following up with these patches

On Fri, Aug 25, 2023 at 05:21:25PM +0200, Niklas Schnelle wrote:
> Pull out the sync operation from viommu_map_pages() by implementing
> ops->iotlb_sync_map. This allows the common IOMMU code to map multiple
> elements of an sg with a single sync (see iommu_map_sg()). Furthermore,
> it is also a requirement for IOMMU_CAP_DEFERRED_FLUSH.
> 
> Link: https://lore.kernel.org/lkml/20230726111433.1105665-1-schnelle@linux.ibm.com/
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> ---
>  drivers/iommu/virtio-iommu.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> index 3551ed057774..fb73dec5b953 100644
> --- a/drivers/iommu/virtio-iommu.c
> +++ b/drivers/iommu/virtio-iommu.c
> @@ -843,7 +843,7 @@ static int viommu_map_pages(struct iommu_domain *domain, unsigned long iova,
>  			.flags		= cpu_to_le32(flags),
>  		};
>  
> -		ret = viommu_send_req_sync(vdomain->viommu, &map, sizeof(map));
> +		ret = viommu_add_req(vdomain->viommu, &map, sizeof(map));
>  		if (ret) {
>  			viommu_del_mappings(vdomain, iova, end);
>  			return ret;
> @@ -909,9 +909,21 @@ static void viommu_iotlb_sync(struct iommu_domain *domain,
>  {
>  	struct viommu_domain *vdomain = to_viommu_domain(domain);
>  
> +	if (!vdomain->nr_endpoints)
> +		return;

I was wondering about these nr_endpoints checks, which seemed unnecessary:
if map()/unmap() were called with no attached endpoints, then no requests
were added to the queue, and viommu_sync_req() below is a nop.

But at least viommu_iotlb_sync_map() and viommu_flush_iotlb_all() need to
handle being called before the domain is finalized (for example by
iommu_create_device_direct_mappings()). In that case vdomain->viommu is
NULL so if you add a NULL check in viommu_sync_req() then you should be
able to drop the nr_endpoints checks in both patches.

Thanks,
Jean

>  	viommu_sync_req(vdomain->viommu);
>  }
>  
> +static int viommu_iotlb_sync_map(struct iommu_domain *domain,
> +				 unsigned long iova, size_t size)
> +{
> +	struct viommu_domain *vdomain = to_viommu_domain(domain);
> +
> +	if (!vdomain->nr_endpoints)
> +		return 0;
> +	return viommu_sync_req(vdomain->viommu);
> +}
> +
>  static void viommu_get_resv_regions(struct device *dev, struct list_head *head)
>  {
>  	struct iommu_resv_region *entry, *new_entry, *msi = NULL;
> @@ -1058,6 +1070,7 @@ static struct iommu_ops viommu_ops = {
>  		.unmap_pages		= viommu_unmap_pages,
>  		.iova_to_phys		= viommu_iova_to_phys,
>  		.iotlb_sync		= viommu_iotlb_sync,
> +		.iotlb_sync_map		= viommu_iotlb_sync_map,
>  		.free			= viommu_domain_free,
>  	}
>  };
> 
> -- 
> 2.39.2
>