Message-ID: <44fdaba0-15eb-0778-bacf-3937a36b0841@gmail.com>
Date:   Thu, 20 Oct 2022 21:11:46 +0300
From:   Xenia Ragiadakou <burzalodowa@...il.com>
To:     Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>
Cc:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <olekstysh@...il.com>
Subject: Re: [PATCH V2] xen/virtio: Handle PCI devices which Host controller
 is described in DT

On 10/20/22 17:12, Oleksandr Tyshchenko wrote:
> 
> On 20.10.22 11:24, Xenia Ragiadakou wrote:
>> On 10/19/22 22:41, Oleksandr Tyshchenko wrote:
>>
>> Hi Oleksandr
> 
> 
> Hello Xenia
> 
> 
>>
>>>
>>> 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@...m.com>
>>>>>>
>>>>>> 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@...m.com>
>>>>>
>>>>> 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@gmail.com/__;!!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);
>>> }
>>>
>>
>> You are right. I prefer your version instead of the above.
> 
> 
> ok, thanks
> 
> 
>>
>>
>>>
>>> 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?
>>>
>>
>> I was thinking passing the device_node along with the device in the
>> function arguments. More specifically, of doing this (not tested, just
>> an idea):
>>
>> bool xen_is_grant_dma_device(struct device *dev)
>> {
>>      struct device_node *np;
>>      bool has_iommu = false;
>>
>>      /* XXX Handle only DT devices for now */
>>      np = xen_dt_get_node(dev);
>>      if (np)
>>          has_iommu = xen_is_dt_grant_dma_device(dev, np);
>>      of_node_put(np);
>>      return has_iommu;
>> }
>>
>> static bool xen_is_dt_grant_dma_device(struct device *dev,
>>                                         struct device_node *np)
>> {
>>      struct device_node *iommu_np = NULL;
>>      bool has_iommu;
>>
>>      if (dev_is_pci(dev)) {
>>          struct pci_dev *pdev = to_pci_dev(dev);
>>      u32 id = PCI_DEVID(pdev->bus->number, pdev->devfn);
>>          of_map_id(np, id, "iommu-map", "iommu-map-mask", &iommu_np,
>> NULL);
>>      } else {
>>          iommu_np = of_parse_phandle(np, "iommus", 0);
>>      }
>>
>>      has_iommu = iommu_np && of_device_is_compatible(iommu_np,
>> "xen,grant-dma");
>>      of_node_put(iommu_np);
>>
>>      return has_iommu;
>> }
> 
> 
> I got it.
> 
> xen_is_grant_dma_device() for V3 won't call xen_dt_get_node(), but call
> xen_is_dt_grant_dma_device() directly.
> 
> static bool xen_is_dt_grant_dma_device(struct device *dev)
> {
>       struct device_node *iommu_np = NULL;
>       bool has_iommu;
> 
>       if (dev_is_pci(dev)) {
>           if (xen_dt_map_id(dev, &iommu_np, NULL))
>               return false;
>       } else if (dev->of_node)
>           iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
>       else
>           return false;
> 
>       has_iommu = iommu_np &&
>               of_device_is_compatible(iommu_np, "xen,grant-dma");
>       of_node_put(iommu_np);
> 
>       return has_iommu;
> }
> 
> bool xen_is_grant_dma_device(struct device *dev)
> {
>       /* XXX Handle only DT devices for now */
>       return xen_is_dt_grant_dma_device(dev);
> }
> 
> 

Ok. One difference, that I see from the previous, is that here you don't 
use the dynamic interface when you access the dev->of_node 
(of_node_get/of_node_put). Before, this was guarded through the external 
xen_dt_get_node().

I suspect that the same needs to be done for the function 
xen_grant_setup_dma_ops(). There, also, the code walks up to the root 
bus twice.

>>
>> I 'm wondering ... is it possible for the host bridge device node to
>> have the iommus property set? meaning that all of its pci devs will
>> have the same backend?
> 
> Good question. I think, it is possible... This is technically what V1 is
> doing.
> 
> 
> Are you asking because to support "iommus" for PCI devices as well to
> describe that use-case with all PCI devices having the same endpoint ID
> (backend ID)?
> If yes, I think, this could be still described by "iommu-map" property,
> something like that (if we don't want to describe mapping for each PCI
> device one-by-one).
> 
> iommu-map = <0x0 &iommu X 0x1>;
> 
> iommu-map-mask = <0x0>;
> 
> where the X is backend ID.
> 
> 
> It feels to me that it should be written down somewhere that for
> platform devices we expect "iommus" and for PCI devices we expect
> "iommu-map/iommu-map-mask" to be present.

Thanks for the clarification, now I got it. Yes I agree.

>>
>>
>>>>
>>>>>
>>>>>>         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
>>>>>>
>>>>>
>>>>
>>

-- 
Xenia

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