[Qemu-stable] [PATCH] pci-assign: Enable MSIX on device to match guest

[Alex Williamson]

When a guest enables MSIX on a device we evaluate the MSIX vector
table, typically find no unmasked vectors and don't switch the device
to MSIX mode.  This generally works fine and the device will be
switched once the guest enables and therefore unmasks a vector.
Unfortunately some drivers enable MSIX, then use interfaces to send
commands between VF & PF or PF & firmware that act based on the host
state of the device.  These therefore break when MSIX is managed
lazily.  This change re-enables the previous test used to enable MSIX
(see qemu-kvm a6b402c9), which basically guesses whether a vector
will be used based on the data field of the vector table.

Cc: address@hidden
Signed-off-by: Alex Williamson <address@hidden>
---
 hw/kvm/pci-assign.c |   17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

I think we might be able to do a little better than this, but I think
this is the right fix for stable and we can build on it to perhaps only
enable a single vector.

diff --git a/hw/kvm/pci-assign.c b/hw/kvm/pci-assign.c
index e80dad0..12a219b 100644
--- a/hw/kvm/pci-assign.c
+++ b/hw/kvm/pci-assign.c
@@ -1025,6 +1025,19 @@ static bool assigned_dev_msix_masked(MSIXTableEntry 
*entry)
     return (entry->ctrl & cpu_to_le32(0x1)) != 0;
 }
 
+/*
+ * When MSI-X is first enabled the vector table typically has all the
+ * vectors masked, so we can't use that as the obvious test to figure out
+ * how many vectors to initially enable.  Instead we look at the data field
+ * because this is what worked for pci-assign for a long time.  This makes
+ * sure the physical MSI-X state tracks the guest's view, which is important
+ * for some VF/PF and PF/fw communication channels.
+ */
+static bool assigned_dev_msix_skipped(MSIXTableEntry *entry)
+{
+    return !entry->data;
+}
+
 static int assigned_dev_update_msix_mmio(PCIDevice *pci_dev)
 {
     AssignedDevice *adev = DO_UPCAST(AssignedDevice, dev, pci_dev);
@@ -1035,7 +1048,7 @@ static int assigned_dev_update_msix_mmio(PCIDevice 
*pci_dev)
 
     /* Get the usable entry number for allocating */
     for (i = 0; i < adev->msix_max; i++, entry++) {
-        if (assigned_dev_msix_masked(entry)) {
+        if (assigned_dev_msix_skipped(entry)) {
             continue;
         }
         entries_nr++;
@@ -1064,7 +1077,7 @@ static int assigned_dev_update_msix_mmio(PCIDevice 
*pci_dev)
     for (i = 0; i < adev->msix_max; i++, entry++) {
         adev->msi_virq[i] = -1;
 
-        if (assigned_dev_msix_masked(entry)) {
+        if (assigned_dev_msix_skipped(entry)) {
             continue;
         }