[PATCH 4/4] PCI: hv: Propagate coherence from VMbus device to PCI device

Michael Kelley
2022-03-17T16:13:52Z

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@microsoft.com>
---
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));
+
put_pcichild(hv_dev);
}
}
@@ -2191,7 +2200,7 @@ static int create_root_hv_pci_bus(struct hv_pcibus_device *hbus)
return error;

pci_lock_rescan_remove();
- hv_pci_assign_numa_node(hbus);
+ hv_pci_assign_properties(hbus);
pci_bus_assign_resources(bridge->bus);
hv_pci_assign_slots(hbus);
pci_bus_add_devices(bridge->bus);
@@ -2458,7 +2467,7 @@ static void pci_devices_present_work(struct work_struct *work)
*/
pci_lock_rescan_remove();
pci_scan_child_bus(hbus->bridge->bus);
- hv_pci_assign_numa_node(hbus);
+ hv_pci_assign_properties(hbus);
hv_pci_assign_slots(hbus);
pci_unlock_rescan_remove();
break;
--
1.8.3.1