kvm/tdx: Ignore memory conversion to shared of unassigned region

TDX requires vMMIO region to be shared.  For KVM, MMIO region is the region
which kvm memslot isn't assigned to (except in-kernel emulation).
qemu has the memory region for vMMIO at each device level.

While OVMF issues MapGPA(to-shared) conservatively on 32bit PCI MMIO
region, qemu doesn't find corresponding vMMIO region because it's before
PCI device allocation and memory_region_find() finds the device region, not
PCI bus region.  It's safe to ignore MapGPA(to-shared) because when guest
accesses those region they use GPA with shared bit set for vMMIO.  Ignore
memory conversion request of non-assigned region to shared and return
success.  Otherwise OVMF is confused and panics there.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20240229063726.610065-35-xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -2923,6 +2923,18 @@ int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private)
     section = memory_region_find(get_system_memory(), start, size);
     mr = section.mr;
     if (!mr) {
+        /*
+         * Ignore converting non-assigned region to shared.
+         *
+         * TDX requires vMMIO region to be shared to inject #VE to guest.
+         * OVMF issues conservatively MapGPA(shared) on 32bit PCI MMIO region,
+         * and vIO-APIC 0xFEC00000 4K page.
+         * OVMF assigns 32bit PCI MMIO region to
+         * [top of low memory: typically 2GB=0xC000000,  0xFC00000)
+         */
+        if (!to_private) {
+            return 0;
+        }
         return -1;
     }