Re: [PATCH V2] xen/virtio: Handle PCI devices which Host controller is described in DT

[Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>]

On 19.10.22 11:47, Xenia Ragiadakou wrote:

Hello Xenia

> On 10/19/22 03:58, Stefano Stabellini wrote:
>> On Sat, 15 Oct 2022, Oleksandr Tyshchenko wrote:
>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>>>
>>> Use the same "xen-grant-dma" device concept for the PCI devices
>>> behind device-tree based PCI Host controller, but with one 
>>> modification.
>>> Unlike for platform devices, we cannot use generic IOMMU bindings
>>> (iommus property), as we need to support more flexible configuration.
>>> The problem is that PCI devices under the single PCI Host controller
>>> may have the backends running in different Xen domains and thus have
>>> different endpoints ID (backend domains ID).
>>>
>>> So use generic PCI-IOMMU bindings instead (iommu-map/iommu-map-mask
>>> properties) which allows us to describe relationship between PCI
>>> devices and backend domains ID properly.
>>>
>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>>
>> Now that I understood the approach and the reasons for it, I can review
>> the patch :-)
>>
>> Please add an example of the bindings in the commit message.
>>
>>
>>> ---
>>> Slightly RFC. This is needed to support Xen grant mappings for 
>>> virtio-pci devices
>>> on Arm at some point in the future. The Xen toolstack side is not 
>>> completely ready yet.
>>> Here, for PCI devices we use more flexible way to pass backend domid 
>>> to the guest
>>> than for platform devices.
>>>
>>> Changes V1 -> V2:
>>>     - update commit description
>>>     - rebase
>>>     - rework to use generic PCI-IOMMU bindings instead of generic 
>>> IOMMU bindings
>>>
>>> Previous discussion is at:
>>> https://urldefense.com/v3/__https://lore.kernel.org/xen-devel/20221006174804.2003029-1-olekstysh@xxxxxxxxx/__;!!GF_29dbcQIUBPA!3-vq7Edm3XfKtD5cnNjnOzDQvuo_XrhJ73yH-nPfqOkGGU0IjLG7R7MR_nAJCAPeOutHRLT44wKYwQwz3SauACie_ZAy$
>>>  
>>> [lore[.]kernel[.]org]
>>>
>>> Based on:
>>> https://urldefense.com/v3/__https://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git/log/?h=for-linus-6.1__;!!GF_29dbcQIUBPA!3-vq7Edm3XfKtD5cnNjnOzDQvuo_XrhJ73yH-nPfqOkGGU0IjLG7R7MR_nAJCAPeOutHRLT44wKYwQwz3SauAEnMDHAq$
>>>  
>>> [git[.]kernel[.]org]
>>> ---
>>>   drivers/xen/grant-dma-ops.c | 87 
>>> ++++++++++++++++++++++++++++++++-----
>>>   1 file changed, 76 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>>> index daa525df7bdc..b79d9d6ce154 100644
>>> --- a/drivers/xen/grant-dma-ops.c
>>> +++ b/drivers/xen/grant-dma-ops.c
>>> @@ -10,6 +10,7 @@
>>>   #include <linux/module.h>
>>>   #include <linux/dma-map-ops.h>
>>>   #include <linux/of.h>
>>> +#include <linux/pci.h>
>>>   #include <linux/pfn.h>
>>>   #include <linux/xarray.h>
>>>   #include <linux/virtio_anchor.h>
>>> @@ -292,12 +293,55 @@ static const struct dma_map_ops 
>>> xen_grant_dma_ops = {
>>>       .dma_supported = xen_grant_dma_supported,
>>>   };
>>>   +static struct device_node *xen_dt_get_pci_host_node(struct device 
>>> *dev)
>>> +{
>>> +    struct pci_dev *pdev = to_pci_dev(dev);
>>> +    struct pci_bus *bus = pdev->bus;
>>> +
>>> +    /* Walk up to the root bus to look for PCI Host controller */
>>> +    while (!pci_is_root_bus(bus))
>>> +        bus = bus->parent;
>>> +
>>> +    return of_node_get(bus->bridge->parent->of_node);
>>> +}
>>
>> It seems silly that we need to walk the hierachy that way, but I
>> couldn't find another way to do it
>>
>>
>>> +static struct device_node *xen_dt_get_node(struct device *dev)
>>> +{
>>> +    if (dev_is_pci(dev))
>>> +        return xen_dt_get_pci_host_node(dev);
>>> +
>>> +    return of_node_get(dev->of_node);
>>> +}
>>> +
>>> +static int xen_dt_map_id(struct device *dev, struct device_node 
>>> **iommu_np,
>>> +             u32 *sid)
>>> +{
>>> +    struct pci_dev *pdev = to_pci_dev(dev);
>>> +    u32 rid = PCI_DEVID(pdev->bus->number, pdev->devfn);
>>> +    struct device_node *host_np;
>>> +    int ret;
>>> +
>>> +    host_np = xen_dt_get_pci_host_node(dev);
>>> +    if (!host_np)
>>> +        return -ENODEV;
>>> +
>>> +    ret = of_map_id(host_np, rid, "iommu-map", "iommu-map-mask", 
>>> iommu_np, sid);
>>> +    of_node_put(host_np);
>>> +    return ret;
>>> +}
>>> +
>>>   static bool xen_is_dt_grant_dma_device(struct device *dev)
>>>   {
>>> -    struct device_node *iommu_np;
>>> +    struct device_node *iommu_np = NULL;
>>>       bool has_iommu;
>>>   -    iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
>>> +    if (dev_is_pci(dev)) {
>>> +        if (xen_dt_map_id(dev, &iommu_np, NULL))
>>> +            return false;
>>> +    } else
>>> +        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);
>>> @@ -307,9 +351,17 @@ static bool xen_is_dt_grant_dma_device(struct 
>>> device *dev)
>>>     bool xen_is_grant_dma_device(struct device *dev)
>>>   {
>>> +    struct device_node *np;
>>> +
>>>       /* XXX Handle only DT devices for now */
>>> -    if (dev->of_node)
>>> -        return xen_is_dt_grant_dma_device(dev);
>>> +    np = xen_dt_get_node(dev);
>>> +    if (np) {
>>> +        bool ret;
>>> +
>>> +        ret = xen_is_dt_grant_dma_device(dev);
>>> +        of_node_put(np);
>>> +        return ret;
>>> +    }
>>
>> We don't need to walk the PCI hierachy twice. Maybe we can add the
>> of_node check directly to xen_is_dt_grant_dma_device?
>>
>
> I think in general we could pass directly the host bridge device if 
> dev_is_pci(dev) (which can be retrieved with 
> pci_get_host_bridge_device(to_pci_dev(dev), and after done with it 
> pci_put_host_bridge_device(phb)).
> So that, xen_is_dt_grant_dma_device() and 
> xen_dt_grant_init_backend_domid() won't need to discover it themselves.
> This will simplify the code.


Good point. I have some remark. Can we use pci_find_host_bridge() 
instead? This way we don't have to add #include "../pci/pci.h", and have 
to drop reference afterwards.

With that xen_dt_get_pci_host_node() will became the following:


static struct device_node *xen_dt_get_pci_host_node(struct device *dev)
{
     struct pci_host_bridge *bridge = 
pci_find_host_bridge(to_pci_dev(dev)->bus);

     return of_node_get(bridge->dev.parent->of_node);
}


With Stefano's suggestion, we won't walk the PCI hierarchy twice when 
executing xen_is_grant_dma_device() for PCI device:

xen_is_grant_dma_device() -> xen_is_dt_grant_dma_device() -> 
xen_dt_map_id() -> xen_dt_get_pci_host_node()


What do you think?


>
>>
>>>       return false;
>>>   }
>>> @@ -325,12 +377,19 @@ bool xen_virtio_mem_acc(struct virtio_device 
>>> *dev)
>>>   static int xen_dt_grant_init_backend_domid(struct device *dev,
>>>                          struct xen_grant_dma_data *data)
>>>   {
>>> -    struct of_phandle_args iommu_spec;
>>> +    struct of_phandle_args iommu_spec = { .args_count = 1 };
>>>   -    if (of_parse_phandle_with_args(dev->of_node, "iommus", 
>>> "#iommu-cells",
>>> -            0, &iommu_spec)) {
>>> -        dev_err(dev, "Cannot parse iommus property\n");
>>> -        return -ESRCH;
>>> +    if (dev_is_pci(dev)) {
>>> +        if (xen_dt_map_id(dev, &iommu_spec.np, iommu_spec.args)) {
>>> +            dev_err(dev, "Cannot translate ID\n");
>>> +            return -ESRCH;
>>> +        }
>>> +    } else {
>>> +        if (of_parse_phandle_with_args(dev->of_node, "iommus", 
>>> "#iommu-cells",
>>> +                0, &iommu_spec)) {
>>> +            dev_err(dev, "Cannot parse iommus property\n");
>>> +            return -ESRCH;
>>> +        }
>>>       }
>>>         if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
>>> @@ -354,6 +413,7 @@ static int 
>>> xen_dt_grant_init_backend_domid(struct device *dev,
>>>   void xen_grant_setup_dma_ops(struct device *dev)
>>>   {
>>>       struct xen_grant_dma_data *data;
>>> +    struct device_node *np;
>>>         data = find_xen_grant_dma_data(dev);
>>>       if (data) {
>>> @@ -365,8 +425,13 @@ void xen_grant_setup_dma_ops(struct device *dev)
>>>       if (!data)
>>>           goto err;
>>>   -    if (dev->of_node) {
>>> -        if (xen_dt_grant_init_backend_domid(dev, data))
>>> +    np = xen_dt_get_node(dev);
>>> +    if (np) {
>>> +        int ret;
>>> +
>>> +        ret = xen_dt_grant_init_backend_domid(dev, data);
>>> +        of_node_put(np);
>>> +        if (ret)
>>>               goto err;
>>>       } else if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT)) {
>>>           dev_info(dev, "Using dom0 as backend\n");
>>> -- 
>>> 2.25.1
>>>
>>
>
-- 
Regards,

Oleksandr Tyshchenko