From: Nicholas Piggin <npiggin@gmail.com>

Of the MSI-X PBA pending bits, the PCI Local Bus Specification says:

  Software should never write, and should only read
  Pending Bits. If software writes to Pending Bits, the
  result is undefined.

Log a GUEST_ERROR message if the PBA is written to by software.

Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: Dmitry Fleytman <dmitry.fleytman@gmail.com>
Cc: Sriram Yagnaraman <sriram.yagnaraman@ericsson.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20250117172842.406338-2-npiggin@gmail.com>
Reviewed-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/pci/msix.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/hw/pci/msix.c b/hw/pci/msix.c
index 57ec7084a4..66f27b9d71 100644
--- a/hw/pci/msix.c
+++ b/hw/pci/msix.c
@@ -15,6 +15,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/log.h"
 #include "hw/pci/msi.h"
 #include "hw/pci/msix.h"
 #include "hw/pci/pci.h"
@@ -260,6 +261,14 @@ static uint64_t msix_pba_mmio_read(void *opaque, hwaddr addr,
 static void msix_pba_mmio_write(void *opaque, hwaddr addr,
                                 uint64_t val, unsigned size)
 {
+    PCIDevice *dev = opaque;
+
+    qemu_log_mask(LOG_GUEST_ERROR,
+                  "PCI [%s:%02x:%02x.%x] attempt to write to MSI-X "
+                  "PBA at 0x%" FMT_PCIBUS ", ignoring.\n",
+                  pci_root_bus_path(dev), pci_dev_bus_num(dev),
+                  PCI_SLOT(dev->devfn), PCI_FUNC(dev->devfn),
+                  addr);
 }
 
 static const MemoryRegionOps msix_pba_mmio_ops = {
-- 
MST