pci-host: designware: add pcie-msi read method

Add pcie-msi mmio read method to avoid NULL pointer dereference
issue.

Reported-by: Lei Sun <slei.casper@gmail.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-Id: <20200811114133.672647-3-ppandit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/pci-host/designware.c

@@ -21,6 +21,7 @@
 #include "qemu/osdep.h"
 #include "qapi/error.h"
 #include "qemu/module.h"
+#include "qemu/log.h"
 #include "hw/pci/msi.h"
 #include "hw/pci/pci_bridge.h"
 #include "hw/pci/pci_host.h"
@@ -63,6 +64,23 @@ designware_pcie_root_to_host(DesignwarePCIERoot *root)
     return DESIGNWARE_PCIE_HOST(bus->parent);
 }
 
+static uint64_t designware_pcie_root_msi_read(void *opaque, hwaddr addr,
+                                              unsigned size)
+{
+    /*
+     * Attempts to read from the MSI address are undefined in
+     * the PCI specifications. For this hardware, the datasheet
+     * specifies that a read from the magic address is simply not
+     * intercepted by the MSI controller, and will go out to the
+     * AHB/AXI bus like any other PCI-device-initiated DMA read.
+     * This is not trivial to implement in QEMU, so since
+     * well-behaved guests won't ever ask a PCI device to DMA from
+     * this address we just log the missing functionality.
+     */
+    qemu_log_mask(LOG_UNIMP, "%s not implemented\n", __func__);
+    return 0;
+}
+
 static void designware_pcie_root_msi_write(void *opaque, hwaddr addr,
                                            uint64_t val, unsigned len)
 {
@@ -77,6 +95,7 @@ static void designware_pcie_root_msi_write(void *opaque, hwaddr addr,
 }
 
 static const MemoryRegionOps designware_pci_host_msi_ops = {
+    .read = designware_pcie_root_msi_read,
     .write = designware_pcie_root_msi_write,
     .endianness = DEVICE_LITTLE_ENDIAN,
     .valid = {