[PATCH v2 2/3] vmd: disable MSI remapping bypass under Xen

[Roger Pau Monne <roger.pau@citrix.com>]

MSI remapping bypass (directly configuring MSI entries for devices on the
VMD bus) won't work under Xen, as Xen is not aware of devices in such bus,
and hence cannot configure the entries using the pIRQ interface in the PV
case, and in the PVH case traps won't be setup for MSI entries for such
devices.

Until Xen is aware of devices in the VMD bus prevent the
VMD_FEAT_CAN_BYPASS_MSI_REMAP capability from being used when running as
any kind of Xen guest.

The MSI remapping bypass is an optional feature of VMD bridges, and hence
when running under Xen it will be masked and devices will be forced to
redirect its interrupts from the VMD bridge.  That mode of operation must
always be supported by VMD bridges and works when Xen is not aware of
devices behind the VMD bridge.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes since v1:
 - Add xen header.
 - Expand comment.
---
 drivers/pci/controller/vmd.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
index 264a180403a0..33c9514bd926 100644
--- a/drivers/pci/controller/vmd.c
+++ b/drivers/pci/controller/vmd.c
@@ -17,6 +17,8 @@
 #include <linux/rculist.h>
 #include <linux/rcupdate.h>
 
+#include <xen/xen.h>
+
 #include <asm/irqdomain.h>
 
 #define VMD_CFGBAR	0
@@ -965,6 +967,23 @@ static int vmd_probe(struct pci_dev *dev, const struct pci_device_id *id)
 	struct vmd_dev *vmd;
 	int err;
 
+	if (xen_domain())
+		/*
+		 * Xen doesn't have knowledge about devices in the VMD bus
+		 * because the config space of devices behind the VMD bridge is
+		 * not known to Xen, and hence Xen cannot discover or configure
+		 * them in any way.
+		 *
+		 * Bypass of MSI remapping won't work in that case as direct
+		 * write by Linux to the MSI entries won't result in functional
+		 * interrupts, as it's Xen the entity that manages the host
+		 * interrupt controller and must configure interrupts.
+		 * However multiplexing of interrupts by the VMD bridge will
+		 * work under Xen, so force the usage of that mode which must
+		 * always be supported by VMD bridges.
+		 */
+		features &= ~VMD_FEAT_CAN_BYPASS_MSI_REMAP;
+
 	if (resource_size(&dev->resource[VMD_CFGBAR]) < (1 << 20))
 		return -ENOMEM;
 
-- 
2.46.0