Re: [PATCH V3 7/8] xen/grant-dma-ops: Retrieve the ID of backend's domain for DT devices

[Stefano Stabellini <sstabellini@kernel.org>]

On Tue, 31 May 2022, Oleksandr Tyshchenko wrote:

> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> Use the presence of "iommus" property pointed to the IOMMU node with
> recently introduced "xen,grant-dma" compatible as a clear indicator
> of enabling Xen grant mappings scheme for that device and read the ID
> of Xen domain where the corresponding backend is running. The domid
> (domain ID) is used as an argument to the Xen grant mapping APIs.
> 
> To avoid the deferred probe timeout which takes place after reusing
> generic IOMMU device tree bindings (because the IOMMU device never
> becomes available) enable recently introduced stub IOMMU driver by
> selecting XEN_GRANT_DMA_IOMMU.
> 
> Also introduce xen_is_grant_dma_device() to check whether xen-grant
> DMA ops need to be set for a passed device.
> 
> Remove the hardcoded domid 0 in xen_grant_setup_dma_ops().
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> ---
> Changes RFC -> V1:
>    - new patch, split required changes from commit:
>     "[PATCH 4/6] virtio: Various updates to xen-virtio DMA ops layer"
>    - update checks in xen_virtio_setup_dma_ops() to only support
>      DT devices for now
>    - remove the "virtio,mmio" check from xen_is_virtio_device()
>    - remane everything according to the new naming scheme:
>      s/virtio/grant_dma
> 
> Changes V1 -> V2:
>    - remove dev_is_pci() check in xen_grant_setup_dma_ops()
>    - remove EXPORT_SYMBOL_GPL(xen_is_grant_dma_device);
> 
> Changes V2 -> V3:
>    - Stefano already gave his Reviewed-by, I dropped it due to the changes (significant)
>    - update commit description
>    - reuse generic IOMMU device tree bindings, select XEN_GRANT_DMA_IOMMU
>      to avoid the deferred probe timeout
> ---
>  drivers/xen/Kconfig         |  1 +
>  drivers/xen/grant-dma-ops.c | 48 ++++++++++++++++++++++++++++++++++++++-------
>  include/xen/xen-ops.h       |  5 +++++
>  3 files changed, 47 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index 35d20d9..bfd5f4f 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -347,6 +347,7 @@ config XEN_VIRTIO
>  	bool "Xen virtio support"
>  	depends on VIRTIO
>  	select XEN_GRANT_DMA_OPS
> +	select XEN_GRANT_DMA_IOMMU if OF
>  	help
>  	  Enable virtio support for running as Xen guest. Depending on the
>  	  guest type this will require special support on the backend side
> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> index 44659f4..6586152 100644
> --- a/drivers/xen/grant-dma-ops.c
> +++ b/drivers/xen/grant-dma-ops.c
> @@ -55,11 +55,6 @@ static struct xen_grant_dma_data *find_xen_grant_dma_data(struct device *dev)
>   * Such a DMA address is formed by using the grant reference as a frame
>   * number and setting the highest address bit (this bit is for the backend
>   * to be able to distinguish it from e.g. a mmio address).
> - *
> - * Note that for now we hard wire dom0 to be the backend domain. In order
> - * to support any domain as backend we'd need to add a way to communicate
> - * the domid of this backend, e.g. via Xenstore, via the PCI-device's
> - * config space or DT/ACPI.
>   */
>  static void *xen_grant_dma_alloc(struct device *dev, size_t size,
>  				 dma_addr_t *dma_handle, gfp_t gfp,
> @@ -275,9 +270,26 @@ static const struct dma_map_ops xen_grant_dma_ops = {
>  	.dma_supported = xen_grant_dma_supported,
>  };
>  
> +bool xen_is_grant_dma_device(struct device *dev)
> +{
> +	struct device_node *iommu_np;
> +	bool has_iommu;
> +
> +	/* XXX Handle only DT devices for now */
> +	if (!dev->of_node)
> +		return false;
> +
> +	iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
> +	has_iommu = iommu_np && of_device_is_compatible(iommu_np, "xen,grant-dma");
> +	of_node_put(iommu_np);
> +
> +	return has_iommu;
> +}
> +
>  void xen_grant_setup_dma_ops(struct device *dev)
>  {
>  	struct xen_grant_dma_data *data;
> +	struct of_phandle_args iommu_spec;
>  
>  	data = find_xen_grant_dma_data(dev);
>  	if (data) {
> @@ -285,12 +297,34 @@ void xen_grant_setup_dma_ops(struct device *dev)
>  		return;
>  	}
>  
> +	/* XXX ACPI device unsupported for now */
> +	if (!dev->of_node)
> +		goto err;
> +
> +	if (of_parse_phandle_with_args(dev->of_node, "iommus", "#iommu-cells",
> +			0, &iommu_spec)) {
> +		dev_err(dev, "Cannot parse iommus property\n");
> +		goto err;
> +	}
> +
> +	if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
> +			iommu_spec.args_count != 1) {
> +		dev_err(dev, "Incompatible IOMMU node\n");
> +		of_node_put(iommu_spec.np);
> +		goto err;
> +	}
> +
> +	of_node_put(iommu_spec.np);
> +
>  	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
>  	if (!data)
>  		goto err;
>  
> -	/* XXX The dom0 is hardcoded as the backend domain for now */
> -	data->backend_domid = 0;
> +	/*
> +	 * The endpoint ID here means the ID of the domain where the corresponding
> +	 * backend is running
> +	 */
> +	data->backend_domid = iommu_spec.args[0];
>  
>  	if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
>  			GFP_KERNEL))) {
> diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h
> index 4f9fad5..62be9dc 100644
> --- a/include/xen/xen-ops.h
> +++ b/include/xen/xen-ops.h
> @@ -223,10 +223,15 @@ static inline void xen_preemptible_hcall_end(void) { }
>  
>  #ifdef CONFIG_XEN_GRANT_DMA_OPS
>  void xen_grant_setup_dma_ops(struct device *dev);
> +bool xen_is_grant_dma_device(struct device *dev);
>  #else
>  static inline void xen_grant_setup_dma_ops(struct device *dev)
>  {
>  }
> +static inline bool xen_is_grant_dma_device(struct device *dev)
> +{
> +	return false;
> +}
>  #endif /* CONFIG_XEN_GRANT_DMA_OPS */
>  
>  #endif /* INCLUDE_XEN_OPS_H */
> -- 
> 2.7.4
> 
> 
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