On 24.05.22 04:58, Stefano Stabellini wrote:
Hello Stefano, all
On Mon, 23 May 2022, Oleksandr wrote:
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>;
};
Not bad! I like it.
Good.
I am wondering, would be the proper solution to eliminate deferred probe
timeout issue in our particular case (without introducing an extra IOMMU
driver)?
In reality I don't think there is a way to do that. I would create an
empty skelethon IOMMU driver for xen,grant-dma.
Ok, I found yet another option how we can avoid deferred probe timeout
issue. I am not sure whether it will be welcome. But it doesn't really
require introducing stub IOMMU driver or other changes in the guest. The
idea is to make IOMMU device unavailable (status = "disabled"), this way
of_iommu_configure() will treat that as success condition also.
https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L31
https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L149
xen_grant_dma {
compatible = "xen,grant-dma";
#iommu-cells = <0x01>;
phandle = <0xfde9>;
status = "disabled";
};
virtio@2000000 {
compatible = "virtio,mmio";
reg = <0x00 0x2000000 0x00 0x200>;
interrupts = <0x00 0x01 0xf01>;
interrupt-parent = <0xfde8>;
dma-coherent;
iommus = <0xfde9 0x01>;
};
I have checked, this "fixes" deferred probe timeout issue.
Or we indeed need to introduce stub IOMMU driver (I placed it to
driver/xen instead of driver/iommu, also we can even squash it with
grant-dma-ops.c?).
This stub driver also results in NO_IOMMU condition (as "of_xlate"
callback is not implemented).
diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index a7bd8ce..35b91b9 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -335,6 +335,10 @@ config XEN_UNPOPULATED_ALLOC
having to balloon out RAM regions in order to obtain physical
memory
space to create such mappings.
+config XEN_GRANT_DMA_IOMMU
+ bool
+ select IOMMU_API
+
config XEN_GRANT_DMA_OPS
bool
select DMA_OPS
@@ -343,6 +347,7 @@ config XEN_VIRTIO
bool "Xen virtio support"
depends on VIRTIO
select XEN_GRANT_DMA_OPS
+ select XEN_GRANT_DMA_IOMMU
help
Enable virtio support for running as Xen guest. Depending on the
guest type this will require special support on the backend side
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 1a23cb0..c0503f1 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -40,3 +40,4 @@ xen-privcmd-y := privcmd.o
privcmd-buf.o
obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF) += xen-front-pgdir-shbuf.o
obj-$(CONFIG_XEN_UNPOPULATED_ALLOC) += unpopulated-alloc.o
obj-$(CONFIG_XEN_GRANT_DMA_OPS) += grant-dma-ops.o
+obj-$(CONFIG_XEN_GRANT_DMA_IOMMU) += grant-dma-iommu.o
diff --git a/drivers/xen/grant-dma-iommu.c b/drivers/xen/grant-dma-iommu.c
new file mode 100644
index 00000000..b8aad8a
--- /dev/null
+++ b/drivers/xen/grant-dma-iommu.c
@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Stub IOMMU driver which does nothing.
+ * The main purpose of it being present is to reuse generic device-tree
IOMMU
+ * bindings by Xen grant DMA-mapping layer.
+ */
+
+#include <linux/iommu.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+
+struct grant_dma_iommu_device {
+ struct device *dev;
+ struct iommu_device iommu;
+};
+
+/* Nothing is really needed here */
+static const struct iommu_ops grant_dma_iommu_ops;
+
+static const struct of_device_id grant_dma_iommu_of_match[] = {
+ { .compatible = "xen,grant-dma" },
+ { },
+};
+
+static int grant_dma_iommu_probe(struct platform_device *pdev)
+{
+ struct grant_dma_iommu_device *mmu;
+ int ret;
+
+ mmu = devm_kzalloc(&pdev->dev, sizeof(*mmu), GFP_KERNEL);
+ if (!mmu)
+ return -ENOMEM;
+
+ mmu->dev = &pdev->dev;
+
+ ret = iommu_device_register(&mmu->iommu, &grant_dma_iommu_ops,
&pdev->dev);
+ if (ret)
+ return ret;
+
+ platform_set_drvdata(pdev, mmu);
+
+ return 0;
+}
+
+static int grant_dma_iommu_remove(struct platform_device *pdev)
+{
+ struct grant_dma_iommu_device *mmu = platform_get_drvdata(pdev);
+
+ platform_set_drvdata(pdev, NULL);
+ iommu_device_unregister(&mmu->iommu);
+
+ return 0;
+}
+
+static struct platform_driver grant_dma_iommu_driver = {
+ .driver = {
+ .name = "grant-dma-iommu",
+ .of_match_table = grant_dma_iommu_of_match,
+ },
+ .probe = grant_dma_iommu_probe,
+ .remove = grant_dma_iommu_remove,
+};
+
+static int __init grant_dma_iommu_init(void)
+{
+ struct device_node *iommu_np;
+
+ iommu_np = of_find_matching_node(NULL, grant_dma_iommu_of_match);
+ if (!iommu_np)
+ return 0;
+
+ of_node_put(iommu_np);
+
+ return platform_driver_register(&grant_dma_iommu_driver);
+}
+subsys_initcall(grant_dma_iommu_init);
I have checked, this also "fixes" deferred probe timeout issue.
Personally I would prefer the first option, but I would be also happy to
use second option in order to unblock the series.
What do the maintainers think?
--
Regards,
Oleksandr Tyshchenko
