tools: don't expose XENFEAT_hvm_pirqs by default

The HVM pirq feature allows routing interrupts from both physical and emulated
devices over event channels, this was done a performance improvement.  However
its usage is fully undocumented, and the only reference implementation is in
Linux.  It defeats the purpose of local APIC hardware virtualization, because
when using it interrupts avoid the usage of the local APIC altogether.

It has also been reported to not work properly with certain devices, at least
when using some AMD GPUs Linux attempts to route interrupts over event
channels, but Xen doesn't correctly detect such routing, which leads to the
hypervisor complaining with:

(XEN) d15v0: Unsupported MSI delivery mode 7 for Dom15

When MSIs are attempted to be routed over event channels the entry delivery
mode is set to ExtINT, but Xen doesn't detect such routing and attempts to
inject the interrupt following the native MSI path, and the ExtINT delivery
mode is not supported.

Disable HVM PIRQs by default and provide a per-domain option in xl.cfg to
enable such feature.  Also for backwards compatibility keep the feature enabled
for any resumed domains that don't have an explicit selection.

Note that the only user of the feature (Linux) is also able to handle native
interrupts fine, as the feature was already not used if Xen reported local APIC
hardware virtualization active.

Link: https://github.com/QubesOS/qubes-issues/issues/7971
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

tools/python/xen/lowlevel/xc/xc.c

@@ -159,7 +159,9 @@ static PyObject *pyxc_domain_create(XcObject *self,
 
 #if defined (__i386) || defined(__x86_64__)
     if ( config.flags & XEN_DOMCTL_CDF_hvm )
-        config.arch.emulation_flags = (XEN_X86_EMU_ALL & ~XEN_X86_EMU_VPCI);
+        config.arch.emulation_flags = XEN_X86_EMU_ALL &
+                                      ~(XEN_X86_EMU_VPCI |
+                                        XEN_X86_EMU_USE_PIRQ);
 #elif defined (__arm__) || defined(__aarch64__)
     config.arch.gic_version = XEN_DOMCTL_CONFIG_GIC_NATIVE;
 #else