x86/kvm: Reserve KVM_FEATURE_MSI_EXT_DEST_ID

No functional change; just reserve the feature bit for now so that VMMs
can start to implement it.

This will allow the host to indicate that MSI emulation supports 15-bit
destination IDs, allowing up to 32768 CPUs without interrupt remapping.

cf. https://patchwork.kernel.org/patch/11816693/ for qemu
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <4cd59bed05f4b7410d3d1ffd1e997ab53683874d.camel@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/cpuid.rst

@@ -92,6 +92,10 @@ KVM_FEATURE_ASYNC_PF_INT           14          guest checks this feature bit
                                                async pf acknowledgment msr
                                                0x4b564d07.
 
+KVM_FEATURE_MSI_EXT_DEST_ID        15          guest checks this feature bit
+                                               before using extended destination
+                                               ID bits in MSI address bits 11-5.
+
 KVM_FEATURE_CLOCKSOURCE_STABLE_BIT 24          host will warn if no guest-side
                                                per-cpu warps are expected in
                                                kvmclock

arch/x86/include/uapi/asm/kvm_para.h

@@ -32,6 +32,7 @@
 #define KVM_FEATURE_POLL_CONTROL	12
 #define KVM_FEATURE_PV_SCHED_YIELD	13
 #define KVM_FEATURE_ASYNC_PF_INT	14
+#define KVM_FEATURE_MSI_EXT_DEST_ID	15
 
 #define KVM_HINTS_REALTIME      0