Re: [PATCH v2 5/5] iommu/virtio: Support identity-mapped domains

[Eric Auger]

Hi Jean,

On 11/23/21 4:53 PM, Jean-Philippe Brucker wrote:
> Support identity domains for devices that do not offer the
> VIRTIO_IOMMU_F_BYPASS_CONFIG feature, by creating 1:1 mappings between
> the virtual and physical address space. Identity domains created this
> way still perform noticeably better than DMA domains, because they don't
> have the overhead of setting up and tearing down mappings at runtime.
> The performance difference between this and bypass is minimal in
> comparison.
>
> It does not matter that the physical addresses in the identity mappings
> do not all correspond to memory. By enabling passthrough we are trusting
> the device driver and the device itself to only perform DMA to suitable
> locations. In some cases it may even be desirable to perform DMA to MMIO
> regions.
>
> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> ---
>  drivers/iommu/virtio-iommu.c | 63 +++++++++++++++++++++++++++++++++---
>  1 file changed, 58 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> index eceb9281c8c1..6a8a52b4297b 100644
> --- a/drivers/iommu/virtio-iommu.c
> +++ b/drivers/iommu/virtio-iommu.c
> @@ -375,6 +375,55 @@ static size_t viommu_del_mappings(struct viommu_domain *vdomain,
>  	return unmapped;
>  }
>  
> +/*
> + * Fill the domain with identity mappings, skipping the device's reserved
> + * regions.
> + */
> +static int viommu_domain_map_identity(struct viommu_endpoint *vdev,
> +				      struct viommu_domain *vdomain)
> +{
> +	int ret;
> +	struct iommu_resv_region *resv;
> +	u64 iova = vdomain->domain.geometry.aperture_start;
> +	u64 limit = vdomain->domain.geometry.aperture_end;
> +	u32 flags = VIRTIO_IOMMU_MAP_F_READ | VIRTIO_IOMMU_MAP_F_WRITE;
> +	unsigned long granule = 1UL << __ffs(vdomain->domain.pgsize_bitmap);
> +
> +	iova = ALIGN(iova, granule);
> +	limit = ALIGN_DOWN(limit + 1, granule) - 1;
> +
> +	list_for_each_entry(resv, &vdev->resv_regions, list) {
> +		u64 resv_start = ALIGN_DOWN(resv->start, granule);
> +		u64 resv_end = ALIGN(resv->start + resv->length, granule) - 1;
> +
> +		if (resv_end < iova || resv_start > limit)
> +			/* No overlap */
> +			continue;
> +
> +		if (resv_start > iova) {
> +			ret = viommu_add_mapping(vdomain, iova, resv_start - 1,
> +						 (phys_addr_t)iova, flags);
> +			if (ret)
> +				goto err_unmap;
> +		}
> +
> +		if (resv_end >= limit)
> +			return 0;
> +
> +		iova = resv_end + 1;
> +	}
> +
> +	ret = viommu_add_mapping(vdomain, iova, limit, (phys_addr_t)iova,
> +				 flags);
> +	if (ret)
> +		goto err_unmap;
> +	return 0;
> +
> +err_unmap:
> +	viommu_del_mappings(vdomain, 0, iova);
> +	return ret;
> +}
> +
>  /*
>   * viommu_replay_mappings - re-send MAP requests
>   *
> @@ -637,14 +686,18 @@ static int viommu_domain_finalise(struct viommu_endpoint *vdev,
>  	vdomain->viommu		= viommu;
>  
>  	if (domain->type == IOMMU_DOMAIN_IDENTITY) {
> -		if (!virtio_has_feature(viommu->vdev,
> -					VIRTIO_IOMMU_F_BYPASS_CONFIG)) {
> +		if (virtio_has_feature(viommu->vdev,
> +				       VIRTIO_IOMMU_F_BYPASS_CONFIG)) {
> +			vdomain->bypass = true;
> +			return 0;
> +		}
> +
> +		ret = viommu_domain_map_identity(vdev, vdomain);
> +		if (ret) {
>  			ida_free(&viommu->domain_ids, vdomain->id);
> -			vdomain->viommu = 0;
> +			vdomain->viommu = NULL;
nit: that change could have been done in patch 2
>  			return -EOPNOTSUPP;
>  		}
> -
> -		vdomain->bypass = true;
>  	}
>  
>  	return 0;
Besides
Reviewed-by: Eric Auger <eric.auger@xxxxxxxxxx>

Eric