x86/emul: dump unhandled memory accesses for PVH dom0

A PV dom0 can map any host memory as long as it's allowed by the IO
capability range in d->iomem_caps.  On the other hand, a PVH dom0 has no
way to populate MMIO region onto it's p2m, so it's limited to what Xen
initially populates on the p2m based on the host memory map and the enabled
device BARs.

Introduce a new debug build only printk that reports attempts by dom0 to
access addresses not populated on the p2m, and not handled by any emulator.
This is for information purposes only, but might allow getting an idea of
what MMIO ranges might be missing on the p2m.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: 43d8a80a0c
master date: 2025-03-05 10:26:46 +0100

xen/arch/x86/hvm/emulate.c

@@ -337,6 +337,9 @@ static int hvmemul_do_io(
         /* If there is no suitable backing DM, just ignore accesses */
         if ( !s )
         {
+            if ( is_mmio && is_hardware_domain(currd) )
+                gdprintk(XENLOG_DEBUG, "unhandled memory %s %#lx size %u\n",
+                         dir ? "read from" : "write to", addr, size);
             rc = hvm_process_io_intercept(&null_handler, &p);
             vio->req.state = STATE_IOREQ_NONE;
         }