[PATCH 2/2] viridian: allow vCPU hotplug for Windows VMs

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

If Viridian extensions are enabled, Windows wouldn't currently allow
a hotplugged vCPU to be brought up dynamically. We need to expose a special
bit to let the guest know we allow it. It appears we can just start exposing
it without worrying too much about compatibility - see relevant QEMU
discussion here:

https://patchwork.kernel.org/project/qemu-devel/patch/1455364815-19586-1-git-send-email-den@xxxxxxxxxx/

Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
---
 xen/arch/x86/hvm/viridian/viridian.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/viridian/viridian.c 
b/xen/arch/x86/hvm/viridian/viridian.c
index ae1ea86..76e8291 100644
--- a/xen/arch/x86/hvm/viridian/viridian.c
+++ b/xen/arch/x86/hvm/viridian/viridian.c
@@ -76,6 +76,7 @@ typedef union _HV_CRASH_CTL_REG_CONTENTS
 } HV_CRASH_CTL_REG_CONTENTS;
 
 /* Viridian CPUID leaf 3, Hypervisor Feature Indication */
+#define CPUID3D_CPU_DYNAMIC_PARTITIONING (1 << 3)
 #define CPUID3D_CRASH_MSRS (1 << 10)
 #define CPUID3D_SINT_POLLING (1 << 17)
 
@@ -179,8 +180,11 @@ void cpuid_viridian_leaves(const struct vcpu *v, uint32_t 
leaf,
         res->a = u.lo;
         res->b = u.hi;
 
+        /* Expose ability to bring up VPs dynamically - allows vCPU hotplug */
+        res->d = CPUID3D_CPU_DYNAMIC_PARTITIONING;
+
         if ( viridian_feature_mask(d) & HVMPV_crash_ctl )
-            res->d = CPUID3D_CRASH_MSRS;
+            res->d |= CPUID3D_CRASH_MSRS;
         if ( viridian_feature_mask(d) & HVMPV_synic )
             res->d |= CPUID3D_SINT_POLLING;
 
-- 
2.7.4