On 19.05.22 09:03, Oleksandr wrote:
Hello Stefano, all
On 19.05.22 04:06, Stefano Stabellini wrote:
Hello Stefano, all
On Thu, 19 May 2022, Oleksandr wrote:
On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@xxxxxxxxx> wrote:
On 18.05.22 17:32, Arnd Bergmann wrote:
On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
<olekstysh@xxxxxxxxx> wrote:
This would mean having a device
node for the grant-table mechanism that can be referred to using the
'iommus'
phandle property, with the domid as an additional argument.
I assume, you are speaking about something like the following?
xen_dummy_iommu {
compatible = "xen,dummy-iommu";
#iommu-cells = <1>;
};
virtio@3000 {
compatible = "virtio,mmio";
reg = <0x3000 0x100>;
interrupts = <41>;
/* The device is located in Xen domain with ID 1 */
iommus = <&xen_dummy_iommu 1>;
};
Right, that's that's the idea,
thank you for the confirmation
except I would not call it a 'dummy'.
From the perspective of the DT, this behaves just like an IOMMU,
even if the exact mechanism is different from most hardware IOMMU
implementations.
well, agree
It does not quite fit the model that Linux currently uses for
iommus,
as that has an allocator for dma_addr_t space
yes (# 3/7 adds grant-table based allocator)
, but it would think it's
conceptually close enough that it makes sense for the binding.
Interesting idea. I am wondering, do we need an extra actions for
this
to work in Linux guest (dummy IOMMU driver, etc)?
It depends on how closely the guest implementation can be made to
resemble a normal iommu. If you do allocate dma_addr_t addresses,
it may actually be close enough that you can just turn the grant-table
code into a normal iommu driver and change nothing else.
Unfortunately, I failed to find a way how use grant references at the
iommu_ops level (I mean to fully pretend that we are an IOMMU
driver). I am
not too familiar with that, so what is written below might be wrong
or at
least not precise.
The normal IOMMU driver in Linux doesn’t allocate DMA addresses by
itself, it
just maps (IOVA-PA) what was requested to be mapped by the upper
layer. The
DMA address allocation is done by the upper layer (DMA-IOMMU which
is the glue
layer between DMA API and IOMMU API allocates IOVA for PA?). But,
all what we
need here is just to allocate our specific grant-table based DMA
addresses
(DMA address = grant reference + offset in the page), so let’s say
we need an
entity to take a physical address as parameter and return a DMA
address (what
actually commit #3/7 is doing), and that’s all. So working at the
dma_ops
layer we get exactly what we need, with the minimal changes to guest
infrastructure. In our case the Xen itself acts as an IOMMU.
Assuming that we want to reuse the IOMMU infrastructure somehow for
our needs.
I think, in that case we will likely need to introduce a new
specific IOVA
allocator (alongside with a generic one) to be hooked up by the
DMA-IOMMU
layer if we run on top of Xen. But, even having the specific IOVA
allocator to
return what we indeed need (DMA address = grant reference + offset
in the
page) we will still need the specific minimal required IOMMU driver
to be
present in the system anyway in order to track the mappings(?) and
do nothing
with them, returning a success (this specific IOMMU driver should
have all
mandatory callbacks implemented).
I completely agree, it would be really nice to reuse generic IOMMU
bindings
rather than introducing Xen specific property if what we are trying to
implement in current patch series fits in the usage of "iommus" in
Linux
more-less. But, if we will have to add more complexity/more
components to the
code for the sake of reusing device tree binding, this raises a
question
whether that’s worthwhile.
Or I really missed something?
I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
bindings, not necessarily the IOMMU drivers framework in Linux (although
that would be an added bonus.)
I know from previous discussions with you that making the grant table
fit in the existing IOMMU drivers model is difficult, but just reusing
the Device Tree bindings seems feasible?
I started experimenting with that. As wrote in a separate email, I got
a deferred probe timeout,
after inserting required nodes into guest device tree, which seems to
be a consequence of the unavailability of IOMMU, I will continue to
investigate this question.
I have experimented with that. Yes, just reusing the Device Tree
bindings is technically feasible (and we are able to do this by only
touching grant-dma-ops.c), although deferred probe timeout still stands
(as there is no IOMMU driver being present actually).
[ 0.583771] virtio-mmio 2000000.virtio: deferred probe timeout,
ignoring dependency
[ 0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks
(2.10 GB/1.95 GiB)
Below the working diff (on top of current series):
diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
index da9c7ff..6586152 100644
--- a/drivers/xen/grant-dma-ops.c
+++ b/drivers/xen/grant-dma-ops.c
@@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops = {
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;
- return of_property_read_bool(dev->of_node, "xen,backend-domid");
+ 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;
- uint32_t domid;
+ struct of_phandle_args iommu_spec;
data = find_xen_grant_dma_data(dev);
if (data) {
@@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
if (!dev->of_node)
goto err;
- if (of_property_read_u32(dev->of_node, "xen,backend-domid",
&domid)) {
- dev_err(dev, "xen,backend-domid property is not present\n");
+ 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;
- data->backend_domid = domid;
+ /*
+ * 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))) {
(END)
Below, the nodes generated by Xen toolstack:
xen_grant_dma {
compatible = "xen,grant-dma";
#iommu-cells = <0x01>;
phandle = <0xfde9>;
};
virtio@2000000 {
compatible = "virtio,mmio";
reg = <0x00 0x2000000 0x00 0x200>;
interrupts = <0x00 0x01 0xf01>;
interrupt-parent = <0xfde8>;
dma-coherent;
iommus = <0xfde9 0x01>;
};
I am wondering, would be the proper solution to eliminate deferred probe
timeout issue in our particular case (without introducing an extra IOMMU
driver)?
--
Regards,
Oleksandr Tyshchenko
