x86/pvh: declare PVH dom0 supported with caveats

PVH dom0 is functionally very similar to PVH domU except for the domain
builder and the added set of hypercalls available to it.

The main concern with declaring it "Supported" is the lack of some features
when compared to classic PV dom0, hence switch it's status to supported with
caveats.  List the known missing features, there might be more features missing
or not working as expected apart from the ones listed.

Note there's some (limited) PVH dom0 testing on both osstest and gitlab.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    - HVM PIRQs are disabled by default.
    - Reduce IOMMU setup time for hardware domain.
    - Allow HVM/PVH domains to map foreign pages.
+   - Declare PVH dom0 supported with caveats.
  - xl/libxl configures vkb=[] for HVM domains with priority over vkb_device.
  - Increase the maximum number of CPUs Xen can be built for from 4095 to
    16383.

SUPPORT.md

@@ -161,7 +161,20 @@ Requires hardware virtualisation support (Intel VMX / AMD SVM).
 Dom0 support requires an IOMMU (Intel VT-d / AMD IOMMU).
 
     Status, domU: Supported
-    Status, dom0: Experimental
+    Status, dom0: Supported, with caveats
+
+PVH dom0 hasn't received the same test coverage as PV dom0, so it can exhibit
+unexpected behavior or issues on some hardware.
+
+At least the following features are missing on a PVH dom0:
+
+  * PCI SR-IOV and Resizable BARs.
+
+  * Native NMI forwarding (nmi=dom0 command line option).
+
+  * MCE handling.
+
+  * PCI Passthrough to any kind of domUs.
 
 ### ARM
 

xen/arch/x86/hvm/dom0_build.c

@@ -1360,7 +1360,6 @@ int __init dom0_construct_pvh(struct domain *d, const module_t *image,
         print_e820_memory_map(d->arch.e820, d->arch.nr_e820);
     }
 
-    printk("WARNING: PVH is an experimental mode with limited functionality\n");
     return 0;
 }