On 2022-03-18 05:12, Michael Kelley (LINUX) wrote:
From: Robin Murphy <robin.murphy@xxxxxxx> Sent: Thursday, March 17, 2022 10:15 AM
On 2022-03-17 16:25, Michael Kelley via iommu wrote:
PCI pass-thru devices in a Hyper-V VM are represented as a VMBus
device and as a PCI device. The coherence of the VMbus device is
set based on the VMbus node in ACPI, but the PCI device has no
ACPI node and defaults to not hardware coherent. This results
in extra software coherence management overhead on ARM64 when
devices are hardware coherent.
Fix this by propagating the coherence of the VMbus device to the
PCI device. There's no effect on x86/x64 where devices are
always hardware coherent.
Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
---
drivers/pci/controller/pci-hyperv.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index ae0bc2f..14276f5 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -49,6 +49,7 @@
#include <linux/refcount.h>
#include <linux/irqdomain.h>
#include <linux/acpi.h>
+#include <linux/dma-map-ops.h>
#include <asm/mshyperv.h>
/*
@@ -2142,9 +2143,9 @@ static void hv_pci_remove_slots(struct hv_pcibus_device
*hbus)
}
/*
- * Set NUMA node for the devices on the bus
+ * Set NUMA node and DMA coherence for the devices on the bus
*/
-static void hv_pci_assign_numa_node(struct hv_pcibus_device *hbus)
+static void hv_pci_assign_properties(struct hv_pcibus_device *hbus)
{
struct pci_dev *dev;
struct pci_bus *bus = hbus->bridge->bus;
@@ -2167,6 +2168,14 @@ static void hv_pci_assign_numa_node(struct
hv_pcibus_device *hbus)
numa_map_to_online_node(
hv_dev->desc.virtual_numa_node));
+ /*
+ * On ARM64, propagate the DMA coherence from the VMbus device
+ * to the corresponding PCI device. On x86/x64, these calls
+ * have no effect because DMA is always hardware coherent.
+ */
+ dev_set_dma_coherent(&dev->dev,
+ dev_is_dma_coherent(&hbus->hdev->device));
Eww... if you really have to do this, I'd prefer to see a proper
hv_dma_configure() helper implemented and wired up to
pci_dma_configure(). Although since it's a generic property I guess at
worst pci_dma_configure could perhaps propagate coherency from the host
bridge to its children by itself in the absence of any other firmware
info. And it's built-in so could use arch_setup_dma_ops() like everyone
else.
I'm not seeing an existing mechanism to provide a "helper" or override
of pci_dma_configure(). Could you elaborate? Or is this something
that needs to be created?
I mean something like the diff below (other #includes omitted for
clarity). Essentially if VMBus has its own way of describing parts of
the system, then for those parts it's nice if it could fit into the same
abstractions we use for firmware-based system description.
Cheers,
Robin.
----->8-----
diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index 588588cfda48..7d92ccad1569 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -20,6 +20,7 @@
#include <linux/of_device.h>
#include <linux/acpi.h>
#include <linux/dma-map-ops.h>
+#include <linux/hyperv.h>
#include "pci.h"
#include "pcie/portdrv.h"
@@ -1602,6 +1603,8 @@ static int pci_dma_configure(struct device *dev)
struct acpi_device *adev = to_acpi_device_node(bridge->fwnode);
ret = acpi_dma_configure(dev, acpi_get_dma_attr(adev));
+ } else if (is_vmbus_dev(bridge)) {
+ ret = hv_dma_configure(dev, device_get_dma_attr(bridge));
}
pci_put_host_bridge_device(bridge);
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index f565a8938836..d1d4dd3d5a3a 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1764,4 +1764,19 @@ static inline unsigned long virt_to_hvpfn(void *addr)
#define HVPFN_DOWN(x) ((x) >> HV_HYP_PAGE_SHIFT)
#define page_to_hvpfn(page) (page_to_pfn(page) * NR_HV_HYP_PAGES_IN_PAGE)
+static inline bool is_vmbus_dev(struct device *dev)
+{
+ /*
+ * dev->bus == &hv_bus would break when the caller is built-in
+ * and CONFIG_HYPERV=m, so look for it by name instead.
+ */
+ return !strcmp(dev->bus->name, "vmbus");
+}
+
+static inline int hv_dma_configure(struct device *dev, enum
dev_dma_attr attr)
+{
+ arch_setup_dma_ops(dev, 0, 0, NULL, attr == DEV_DMA_COHERENT);
+ return 0;
+}
+
#endif /* _HYPERV_H */
