[PATCH 1/3] hw/cxl: Add stub write function for RO MemoryRegionOps entries.

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

There is no checking on the availability of a write callback.
Hence QEMU crashes if a write does occur to one of these regions.

Discovered whilst chasing a Linux kernel bug that incorrectly
wrote into one of these regions.

Fixes: 6364adacdf ("hw/cxl/device: Implement the CAP array (8.2.8.1-2)")
Reported-by: Bobo WL <lmw.bobo@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
---
 hw/cxl/cxl-device-utils.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/hw/cxl/cxl-device-utils.c b/hw/cxl/cxl-device-utils.c
index 687759b301..83ce7a8270 100644
--- a/hw/cxl/cxl-device-utils.c
+++ b/hw/cxl/cxl-device-utils.c
@@ -141,9 +141,15 @@ static uint64_t mdev_reg_read(void *opaque, hwaddr offset, unsigned size)
     return retval;
 }
 
+static void ro_reg_write(void *opaque, hwaddr offset, uint64_t value,
+                           unsigned size)
+{
+    /* Many register sets are read only */
+}
+
 static const MemoryRegionOps mdev_ops = {
     .read = mdev_reg_read,
-    .write = NULL, /* memory device register is read only */
+    .write = ro_reg_write,
     .endianness = DEVICE_LITTLE_ENDIAN,
     .valid = {
         .min_access_size = 1,
@@ -173,7 +179,7 @@ static const MemoryRegionOps mailbox_ops = {
 
 static const MemoryRegionOps dev_ops = {
     .read = dev_reg_read,
-    .write = NULL, /* status register is read only */
+    .write = ro_reg_write,
     .endianness = DEVICE_LITTLE_ENDIAN,
     .valid = {
         .min_access_size = 1,
@@ -188,7 +194,7 @@ static const MemoryRegionOps dev_ops = {
 
 static const MemoryRegionOps caps_ops = {
     .read = caps_reg_read,
-    .write = NULL, /* caps registers are read only */
+    .write = ro_reg_write,
     .endianness = DEVICE_LITTLE_ENDIAN,
     .valid = {
         .min_access_size = 1,
-- 
2.32.0