[PULL 06/11] i386: HV_HYPERCALL_AVAILABLE privilege bit is always needed

[Eduardo Habkost]

From: Vitaly Kuznetsov <vkuznets@redhat.com>

According to TLFS, Hyper-V guest is supposed to check
HV_HYPERCALL_AVAILABLE privilege bit before accessing
HV_X64_MSR_GUEST_OS_ID/HV_X64_MSR_HYPERCALL MSRs but at least some
Windows versions ignore that. As KVM is very permissive and allows
accessing these MSRs unconditionally, no issue is observed. We may,
however, want to tighten the checks eventually. Conforming to the
spec is probably also a good idea.

Enable HV_HYPERCALL_AVAILABLE bit unconditionally.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210608120817.1325125-8-vkuznets@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 target/i386/kvm/kvm.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 945d24300c0..eee1a6b46ea 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -813,8 +813,6 @@ static struct {
     [HYPERV_FEAT_RELAXED] = {
         .desc = "relaxed timing (hv-relaxed)",
         .flags = {
-            {.func = HV_CPUID_FEATURES, .reg = R_EAX,
-             .bits = HV_HYPERCALL_AVAILABLE},
             {.func = HV_CPUID_ENLIGHTMENT_INFO, .reg = R_EAX,
              .bits = HV_RELAXED_TIMING_RECOMMENDED}
         }
@@ -823,7 +821,7 @@ static struct {
         .desc = "virtual APIC (hv-vapic)",
         .flags = {
             {.func = HV_CPUID_FEATURES, .reg = R_EAX,
-             .bits = HV_HYPERCALL_AVAILABLE | HV_APIC_ACCESS_AVAILABLE},
+             .bits = HV_APIC_ACCESS_AVAILABLE},
             {.func = HV_CPUID_ENLIGHTMENT_INFO, .reg = R_EAX,
              .bits = HV_APIC_ACCESS_RECOMMENDED}
         }
@@ -832,8 +830,7 @@ static struct {
         .desc = "clocksources (hv-time)",
         .flags = {
             {.func = HV_CPUID_FEATURES, .reg = R_EAX,
-             .bits = HV_HYPERCALL_AVAILABLE | HV_TIME_REF_COUNT_AVAILABLE |
-             HV_REFERENCE_TSC_AVAILABLE}
+             .bits = HV_TIME_REF_COUNT_AVAILABLE | HV_REFERENCE_TSC_AVAILABLE}
         }
     },
     [HYPERV_FEAT_CRASH] = {
@@ -1346,6 +1343,9 @@ static int hyperv_fill_cpuids(CPUState *cs,
     c->ebx = hv_build_cpuid_leaf(cs, HV_CPUID_FEATURES, R_EBX);
     c->edx = hv_build_cpuid_leaf(cs, HV_CPUID_FEATURES, R_EDX);
 
+    /* Unconditionally required with any Hyper-V enlightenment */
+    c->eax |= HV_HYPERCALL_AVAILABLE;
+
     /* Not exposed by KVM but needed to make CPU hotplug in Windows work */
     c->edx |= HV_CPU_DYNAMIC_PARTITIONING_AVAILABLE;
 
-- 
2.31.1