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

[Oleksandr Tyshchenko]

On 07.10.22 04:14, Stefano Stabellini wrote:

Hello Stefano

Thank you for the detailed analysis. Please see answers below.


> On Thu, 6 Oct 2022, Oleksandr Tyshchenko wrote:
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>>
>> Use the same "xen-grant-dma" device concept (based on generic IOMMU
>> device-tree bindings) for the PCI devices behind device-tree based
>> PCI Host controller.
>>
>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>> ---
>> 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 published yet.
>> Here, for PCI devices we use the same way to pass backend domid to the guest as for
>> platform devices.
>>
>> Depends on Juergen's series:
>> https://urldefense.com/v3/__https://lore.kernel.org/xen-devel/20221006071500.15689-1-jgross@xxxxxxxx/__;!!GF_29dbcQIUBPA!waOk2Goc7qlhNo5-csRObryil_GzMF_e61EJR501oJ08cH2dnJulsZXWlelBDTBqa63TVoUcWQTB5NecJ1p4xFNgh2_EuA$  [lore[.]kernel[.]org]
>> ---
>>   drivers/xen/grant-dma-ops.c | 51 +++++++++++++++++++++++++++++--------
>>   1 file changed, 41 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>> index ff9be3aff87e..79d13122ec08 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>
>> @@ -273,12 +274,28 @@ static const struct dma_map_ops xen_grant_dma_ops = {
>>   	.dma_supported = xen_grant_dma_supported,
>>   };
>>   
>> -static bool xen_is_dt_grant_dma_device(struct device *dev)
>> +static struct device_node *xen_dt_get_node(struct device *dev)
>> +{
>> +	if (dev_is_pci(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);
>> +	}
> Is it possible to have multiple virtio devices under a single virtio-pci
> root complex?

yes


> What if virtio-net has the backend in dom0 and
> virtio-block has the backend in dom1?
>
> Or each virtio PCI device shows up under a different PCI root complex?


Good questions. To be honest, it is not 100% clear to me yet how it is 
supposed to be. But let's guess...

I think that having a PCI Host bridge per virtio-pci device is overkill.

So, I see two options here:
1. We provide PCI Host bridge per backends domain to the guest, so each 
PCI Host bridge covers only virtio-pci devices whose backends are 
running within *the same* domain.
With that we would be able to use property at PCI Host bridge level.

2. We provide only a single PCI Host bridge to the guest, so that single 
PCI Host bridge covers all virtio-pci devices assigned to this guest.
No matter where the corresponding backends are running (the 
virtio-devices under that PCI Host bridge can have the backends in 
different domains).
With that we wouldn’t be able to use property at PCI Host bridge level. 
And we need a more flexible option(s) to be able distinguish between 
virtio-pci devices.

Taking into account that for virtio-pci on Arm we need to emulate a 
specific PCI Host bridge in Xen to intercept the guest PCI config space 
accesses
(detect what PCI device is targeted) and forward them to the appropriate 
backend (IOREQ Server),
it feels to me that we likely need to go with the second option here 
(one PCI host bridge per guest), I may mistake,
but I don’t think that we want to emulate several PCI Host bridges for a 
single guest (more code, more resources, etc).



>
> If we can have multiple virtio PCI devices under a single PCI root
> complex, then I think it would be better to check for a per-device
> property, rather than a single property at the PCI root complex level.

Completely agree.


>
> The first thing that comes to mind is to describe each PCI device under
> the root complex in device tree. Although it is uncommon (usually only
> the PCI root complex is described in device tree), it is possible to
> also describe in device tree all the individual PCI devices under the
> root complex.
>
> Given that the domU device tree is generated by Xen and/or the Xen
> toolstack, it would be easy to arrange for it to happen.

Technically yes. If we decide to provide only a single PCI Host bridge 
to the guest, we will have have to deal with the virtio-pci devices with 
various backend_domid,
so we can consider using more flexible property 
“iommu-map”/”iommu-map-mask” specially introduced for such purposes:
https://www.kernel.org/doc/Documentation/devicetree/bindings/pci/pci-iommu.txt
I think, with that we would be able to describe, i.e that virtio-pci 
device A connects to stream_id (backend_domid) X and virtio-pci device B 
to stream_id Y,
and virtio-pci device C to nothing (so is not required to use grants), 
unless I missed something.

I have looked at it and I don’t see at the moment why the idea wouldn’t 
work, but I haven’t experimented with that yet in such context.


>
> That would solve the issue as far as I can tell, but I worry it might
> not be a good idea because if we rely on the per-device device tree node
> to be present then it becomes harder to implement virtio hotplug
> (Virtio hotplug is important to add dom0less support.)
>
> Let's say that we create a dom0less domU with an emulated PCI root
> complex without any devices under it, then after Dom0 is fully booted,
> we add a virtio-net emulated device. How do we tell the guest what is
> the backend domain id?
>
> Device tree and other firmware tables are not relevant anymore.
>
> We could reuse a PCI config space register to expose the backend id.
> However this solution requires a backend change (QEMU) to expose the
> backend id via an emulated register for each emulated device.
>
> To avoid having to introduce a special config space register in all
> emulated PCI devices (virtio-net, virtio-block, etc) I wonder if we
> could add a special PCI config space register at the emulated PCI Root
> Complex level.
>
> Basically the workflow would be as follow:
>
> - Linux recognizes the PCI Root Complex as a Xen PCI Root Complex
> - Linux writes to special PCI config space register of the Xen PCI Root
>    Complex the PCI device id (basically the BDF)
> - The Xen PCI Root Complex emulated by Xen answers by writing back to
>    the same location the backend id (domid of the backend)
> - Linux reads back the same PCI config space register of the Xen PCI
>    Root Complex and learn the relevant domid
>
> What do you think?


I think the idea sounds indeed interesting and would probably work, but 
would require guest modifications other than just in drivers/xen (and 
likely the specification changes as well).
Which ideally of course should be avoided.
Also I was thinking it would be nice not to diverge much between 
communicating the backend_domid for platform and PCI devices on Arm with 
device tree.

If we managed to re-use generic IOMMU device-tree bindings for 
virtio-mmio, we would likely be able to re-use PCI-IOMMU device-tree 
bindings for virtio-pci,
at least for boot PCI devices (which are known at the domain creation time).
The more, the bindings is already present: 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/iommu/xen,grant-dma.yaml

Regarding the hotplug devices, yes it is a valid use-case which should 
be also supported with virtio-pci, I assume the Xenstore could be 
re-used for that purpose if it would be available.
The Xenstore is available with the usual Dom0 and toolstack, is it 
available with dom0less?




>
> Other ideas welcome!
>
>
>
>> +	return of_node_get(dev->of_node);
>> +}
>> +
>> +static bool xen_is_dt_grant_dma_device(struct device_node *np)
>>   {
>>   	struct device_node *iommu_np;
>>   	bool has_iommu;
>>   
>> -	iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
>> +	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);
>> @@ -288,9 +305,17 @@ 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(np);
>> +		of_node_put(np);
>> +		return ret;
>> +	}
>>   
>>   	return false;
>>   }
>> @@ -303,20 +328,20 @@ bool xen_virtio_mem_acc(struct virtio_device *dev)
>>   	return xen_is_grant_dma_device(dev->dev.parent);
>>   }
>>   
>> -static int xen_dt_grant_setup_dma_ops(struct device *dev,
>> +static int xen_dt_grant_setup_dma_ops(struct device_node *np,
>>   				       struct xen_grant_dma_data *data)
>>   {
>>   	struct of_phandle_args iommu_spec;
>>   
>> -	if (of_parse_phandle_with_args(dev->of_node, "iommus", "#iommu-cells",
>> +	if (of_parse_phandle_with_args(np, "iommus", "#iommu-cells",
>>   			0, &iommu_spec)) {
>> -		dev_err(dev, "Cannot parse iommus property\n");
>> +		pr_err("%s: Cannot parse iommus property\n", np->name);
>>   		return -ESRCH;
>>   	}
>>   
>>   	if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
>>   			iommu_spec.args_count != 1) {
>> -		dev_err(dev, "Incompatible IOMMU node\n");
>> +		pr_err("%s: Incompatible IOMMU node\n", iommu_spec.np->name);
>>   		of_node_put(iommu_spec.np);
>>   		return -ESRCH;
>>   	}
>> @@ -335,6 +360,7 @@ static int xen_dt_grant_setup_dma_ops(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) {
>> @@ -346,8 +372,13 @@ void xen_grant_setup_dma_ops(struct device *dev)
>>   	if (!data)
>>   		goto err;
>>   
>> -	if (dev->of_node) {
>> -		if (xen_dt_grant_setup_dma_ops(dev, data))
>> +	np = xen_dt_get_node(dev);
>> +	if (np) {
>> +		int ret;
>> +
>> +		ret = xen_dt_grant_setup_dma_ops(np, 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