Re: [PATCH 2/3] PCI: Add DMA configuration for virtual platforms

From: Robin Murphy <robin.murphy@arm.com>
To: Jean-Philippe Brucker <jean-philippe@linaro.org>,
	iommu@lists.linux-foundation.org,
	virtualization@lists.linux-foundation.org,
	linux-pci@vger.kernel.org
Cc: kevin.tian@intel.com, mst@redhat.com, sebastien.boeuf@intel.com,
	jacob.jun.pan@intel.com, bhelgaas@google.com,
	jasowang@redhat.com
Subject: Re: [PATCH 2/3] PCI: Add DMA configuration for virtual platforms
Date: Fri, 14 Feb 2020 17:03:16 +0000	[thread overview]
Message-ID: <393cce27-dbed-f075-2a67-9882bed801e7@arm.com> (raw)
In-Reply-To: <20200214160413.1475396-3-jean-philippe@linaro.org>

On 14/02/2020 4:04 pm, Jean-Philippe Brucker wrote:
> Hardware platforms usually describe the IOMMU topology using either
> device-tree pointers or vendor-specific ACPI tables.  For virtual
> platforms that don't provide a device-tree, the virtio-iommu device
> contains a description of the endpoints it manages.  That information
> allows us to probe endpoints after the IOMMU is probed (possibly as late
> as userspace modprobe), provided it is discovered early enough.
> 
> Add a hook to pci_dma_configure(), which returns -EPROBE_DEFER if the
> endpoint is managed by a vIOMMU that will be loaded later, or 0 in any
> other case to avoid disturbing the normal DMA configuration methods.
> When CONFIG_VIRTIO_IOMMU_TOPOLOGY isn't selected, the call to
> virt_dma_configure() is compiled out.
> 
> As long as the information is consistent, platforms can provide both a
> device-tree and a built-in topology, and the IOMMU infrastructure is
> able to deal with multiple DMA configuration methods.

Urgh, it's already been established[1] that having IOMMU setup tied to 
DMA configuration at driver probe time is not just conceptually wrong 
but actually broken, so the concept here worries me a bit. In a world 
where of_iommu_configure() and friends are being called much earlier 
around iommu_probe_device() time, how badly will this fall apart?

Robin.

[1] 
https://lore.kernel.org/linux-iommu/9625faf4-48ef-2dd3-d82f-931d9cf26976@huawei.com/

> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> ---
>   drivers/pci/pci-driver.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index 0454ca0e4e3f..69303a814f21 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -18,6 +18,7 @@
>   #include <linux/kexec.h>
>   #include <linux/of_device.h>
>   #include <linux/acpi.h>
> +#include <linux/virt_iommu.h>
>   #include "pci.h"
>   #include "pcie/portdrv.h"
>   
> @@ -1602,6 +1603,10 @@ static int pci_dma_configure(struct device *dev)
>   	struct device *bridge;
>   	int ret = 0;
>   
> +	ret = virt_dma_configure(dev);
> +	if (ret)
> +		return ret;
> +
>   	bridge = pci_get_host_bridge_device(to_pci_dev(dev));
>   
>   	if (IS_ENABLED(CONFIG_OF) && bridge->parent &&
> 