Commit e8ae0e140c05 ("vfio: Require that devices support DMA cache
coherence") requires IOMMU drivers to advertise
IOMMU_CAP_CACHE_COHERENCY, in order to be used by VFIO. Since VFIO does
not provide to userspace the ability to maintain coherency through cache
invalidations, it requires hardware coherency. Advertise the capability
in order to restore VFIO support.
The meaning of IOMMU_CAP_CACHE_COHERENCY also changed from "IOMMU can
enforce cache coherent DMA transactions" to "IOMMU_CACHE is supported".
While virtio-iommu cannot enforce coherency (of PCIe no-snoop
transactions), it does support IOMMU_CACHE.
We can distinguish different cases of non-coherent DMA:
(1) When accesses from a hardware endpoint are not coherent. The host
would describe such a device using firmware methods ('dma-coherent'
in device-tree, '_CCA' in ACPI), since they are also needed without
a vIOMMU. In this case mappings are created without IOMMU_CACHE.
virtio-iommu doesn't need any additional support. It sends the same
requests as for coherent devices.
(2) When the physical IOMMU supports non-cacheable mappings. Supporting
those would require a new feature in virtio-iommu, new PROBE request
property and MAP flags. Device drivers would use a new API to
discover this since it depends on the architecture and the physical
IOMMU.
(3) When the hardware supports PCIe no-snoop. It is possible for
assigned PCIe devices to issue no-snoop transactions, and the
virtio-iommu specification is lacking any mention of this.
Arm platforms don't necessarily support no-snoop, and those that do
cannot enforce coherency of no-snoop transactions. Device drivers
must be careful about assuming that no-snoop transactions won't end
up cached; see commit e02f5c1bb228 ("drm: disable uncached DMA
optimization for ARM and arm64"). On x86 platforms, the host may or
may not enforce coherency of no-snoop transactions with the physical
IOMMU. But according to the above commit, on x86 a driver which
assumes that no-snoop DMA is compatible with uncached CPU mappings
will also work if the host enforces coherency.
Although these issues are not specific to virtio-iommu, it could be
used to facilitate discovery and configuration of no-snoop. This
would require a new feature bit, PROBE property and ATTACH/MAP
flags.
Cc: stable(a)vger.kernel.org
Fixes: e8ae0e140c05 ("vfio: Require that devices support DMA cache coherence")
Signed-off-by: Jean-Philippe Brucker <jean-philippe(a)linaro.org>
---
Since v2 [1], I tried to refine the commit message.
This fix is needed for v5.19 and v6.0.
I can improve the check once Robin's change [2] is merged:
capable(IOMMU_CAP_CACHE_COHERENCY) could return dev->dma_coherent for
case (1) above.
[1] https://lore.kernel.org/linux-iommu/20220818163801.1011548-1-jean-philippe@…
[2] https://lore.kernel.org/linux-iommu/d8bd8777d06929ad8f49df7fc80e1b9af32a41b…
---
drivers/iommu/virtio-iommu.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
index 08eeafc9529f..80151176ba12 100644
--- a/drivers/iommu/virtio-iommu.c
+++ b/drivers/iommu/virtio-iommu.c
@@ -1006,7 +1006,18 @@ static int viommu_of_xlate(struct device *dev, struct of_phandle_args *args)
return iommu_fwspec_add_ids(dev, args->args, 1);
}
+static bool viommu_capable(enum iommu_cap cap)
+{
+ switch (cap) {
+ case IOMMU_CAP_CACHE_COHERENCY:
+ return true;
+ default:
+ return false;
+ }
+}
+
static struct iommu_ops viommu_ops = {
+ .capable = viommu_capable,
.domain_alloc = viommu_domain_alloc,
.probe_device = viommu_probe_device,
.probe_finalize = viommu_probe_finalize,
--
2.37.1
The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable(a)vger.kernel.org>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 25e9fbf0fd38868a429feabc38abebfc6dbf6542 Mon Sep 17 00:00:00 2001
From: "Isaac J. Manjarres" <isaacmanjarres(a)google.com>
Date: Wed, 17 Aug 2022 11:40:26 -0700
Subject: [PATCH] driver core: Don't probe devices after bus_type.match() probe
deferral
Both __device_attach_driver() and __driver_attach() check the return
code of the bus_type.match() function to see if the device needs to be
added to the deferred probe list. After adding the device to the list,
the logic attempts to bind the device to the driver anyway, as if the
device had matched with the driver, which is not correct.
If __device_attach_driver() detects that the device in question is not
ready to match with a driver on the bus, then it doesn't make sense for
the device to attempt to bind with the current driver or continue
attempting to match with any of the other drivers on the bus. So, update
the logic in __device_attach_driver() to reflect this.
If __driver_attach() detects that a driver tried to match with a device
that is not ready to match yet, then the driver should not attempt to bind
with the device. However, the driver can still attempt to match and bind
with other devices on the bus, as drivers can be bound to multiple
devices. So, update the logic in __driver_attach() to reflect this.
Fixes: 656b8035b0ee ("ARM: 8524/1: driver cohandle -EPROBE_DEFER from bus_type.match()")
Cc: stable(a)vger.kernel.org
Cc: Saravana Kannan <saravanak(a)google.com>
Reported-by: Guenter Roeck <linux(a)roeck-us.net>
Tested-by: Guenter Roeck <linux(a)roeck-us.net>
Tested-by: Linus Walleij <linus.walleij(a)linaro.org>
Reviewed-by: Saravana Kannan <saravanak(a)google.com>
Signed-off-by: Isaac J. Manjarres <isaacmanjarres(a)google.com>
Link: https://lore.kernel.org/r/20220817184026.3468620-1-isaacmanjarres@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh(a)linuxfoundation.org>
diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index a8916d1bfdcb..ec69b43f926a 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -911,6 +911,11 @@ static int __device_attach_driver(struct device_driver *drv, void *_data)
dev_dbg(dev, "Device match requests probe deferral\n");
dev->can_match = true;
driver_deferred_probe_add(dev);
+ /*
+ * Device can't match with a driver right now, so don't attempt
+ * to match or bind with other drivers on the bus.
+ */
+ return ret;
} else if (ret < 0) {
dev_dbg(dev, "Bus failed to match device: %d\n", ret);
return ret;
@@ -1150,6 +1155,11 @@ static int __driver_attach(struct device *dev, void *data)
dev_dbg(dev, "Device match requests probe deferral\n");
dev->can_match = true;
driver_deferred_probe_add(dev);
+ /*
+ * Driver could not match with device, but may match with
+ * another device on the bus.
+ */
+ return 0;
} else if (ret < 0) {
dev_dbg(dev, "Bus failed to match device: %d\n", ret);
return ret;