[Qemu-devel] [PATCH 05/23] hyperv: ensure VP index equal to QEMU cpu_index

[Roman Kagan]

Hyper-V identifies vcpus by the virtual processor (VP) index which is
normally queried by the guest via HV_X64_MSR_VP_INDEX msr.

It has to be owned by QEMU in order to preserve it across migration.

However, the current implementation in KVM doesn't allow to set this
msr, and KVM uses its own notion of VP index.  Fortunately, the way
vcpus are created in QEMU/KVM basically guarantees that the KVM value is
equal to QEMU cpu_index.

So, for back and forward compatibility, attempt to set the msr at vcpu
init time to cpu_index, but ignore the errors; then query the msr value
from KVM and assert that it's equal to cpu_index.  On current kernels
this will work by luck; future ones will accept the value from
userspace.

Signed-off-by: Roman Kagan <address@hidden>
---
 target/i386/hyperv.h |  2 ++
 target/i386/hyperv.c |  5 +++++
 target/i386/kvm.c    | 26 ++++++++++++++++++++++++++
 3 files changed, 33 insertions(+)

diff --git a/target/i386/hyperv.h b/target/i386/hyperv.h
index 0c3b562..35da0b1 100644
--- a/target/i386/hyperv.h
+++ b/target/i386/hyperv.h
@@ -39,4 +39,6 @@ void kvm_hv_sint_route_destroy(HvSintRoute *sint_route);
 
 int kvm_hv_sint_route_set_sint(HvSintRoute *sint_route);
 
+uint32_t hyperv_vp_index(X86CPU *cpu);
+
 #endif
diff --git a/target/i386/hyperv.c b/target/i386/hyperv.c
index 227185c..27de5bc 100644
--- a/target/i386/hyperv.c
+++ b/target/i386/hyperv.c
@@ -16,6 +16,11 @@
 #include "hyperv.h"
 #include "hyperv_proto.h"
 
+uint32_t hyperv_vp_index(X86CPU *cpu)
+{
+    return CPU(cpu)->cpu_index;
+}
+
 int kvm_hv_handle_exit(X86CPU *cpu, struct kvm_hyperv_exit *exit)
 {
     CPUX86State *env = &cpu->env;
diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index 251aa95..eb9cde4 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -610,11 +610,37 @@ static int kvm_arch_set_tsc_khz(CPUState *cs)
     return 0;
 }
 
+static void hyperv_set_vp_index(CPUState *cs)
+{
+    struct {
+        struct kvm_msrs info;
+        struct kvm_msr_entry entries[1];
+    } msr_data;
+    int ret;
+
+    msr_data.info.nmsrs = 1;
+    msr_data.entries[0].index = HV_X64_MSR_VP_INDEX;
+
+    /*
+     * Some kernels don't support setting HV_X64_MSR_VP_INDEX.  However,
+     * they use VP_INDEX equal to cpu_index due to the way vcpus are created.
+     * So ignore the errors from SET_MSRS, but verify that GET_MSRS returns the
+     * expected value.
+     */
+    msr_data.entries[0].data = cs->cpu_index;
+    kvm_vcpu_ioctl(cs, KVM_SET_MSRS, &msr_data);
+    ret = kvm_vcpu_ioctl(cs, KVM_GET_MSRS, &msr_data);
+    assert(ret == 1);
+    assert(msr_data.entries[0].data == cs->cpu_index);
+}
+
 static int hyperv_handle_properties(CPUState *cs)
 {
     X86CPU *cpu = X86_CPU(cs);
     CPUX86State *env = &cpu->env;
 
+    hyperv_set_vp_index(cs);
+
     if (cpu->hyperv_time &&
             kvm_check_extension(cs->kvm_state, KVM_CAP_HYPERV_TIME) <= 0) {
         cpu->hyperv_time = false;
-- 
2.9.4