arch: x86_64: Set GuestPhysAddrSize in CPUID leaf 0x80000008

When generating guest CPUID, we set PhysAddrSize (EAX bits 7:0)
based on the host's physical address bits. On AMD hosts with SME,
get_host_cpu_phys_bits() subtracts the c-bit reduction from this
value, but the code here only writes the result to bits 7:0 and
leaves GuestPhysAddrSize (bits 23:16) at the unmodified host value.
This creates a gap: e.g. PhysAddrSize=43 but GuestPhysAddrSize=48.

Guest firmware that reads GuestPhysAddrSize will see a larger
address space than the VMM provides, and may place PCI BARs beyond
the MMIO bus range. However, Cloud-hypervisor sizes its MMIO bus to
phys_bits.

Fix by setting both PhysAddrSize (bits 7:0) and GuestPhysAddrSize
(bits 23:16) to phys_bits, using mask 0xff00_ff00 instead of
0xffff_ff00.

Signed-off-by: Ruben Hakobyan <hruben@meta.com>

arch/src/x86_64/mod.rs

@@ -808,9 +808,11 @@ fn required_common_cpuid_updates(
                 entry.ecx = leaf.ecx;
                 entry.edx = leaf.edx;
             }
-            // Set CPU physical bits
+            // Set CPU physical bits and guest physical bits
             0x8000_0008 => {
-                entry.eax = (entry.eax & 0xffff_ff00) | (config.phys_bits as u32 & 0xff);
+                entry.eax = (entry.eax & 0xff00_ff00)
+                    | (config.phys_bits as u32 & 0xff)
+                    | ((config.phys_bits as u32 & 0xff) << 16);
             }
             0x4000_0001 => {
                 // Enable KVM_FEATURE_MSI_EXT_DEST_ID. This allows the guest to target