[Qemu-devel] [PATCH] i386: Disable MSR_PLATFORM_INFO emulation

[Eduardo Habkost <ehabkost@redhat.com>]

Linux v4.12 introduced[1] emulation of MSR_PLATFORM_INFO and
MSR_MISC_FEATURES_ENABLES, and enabled the
MSR_PLATFORM_INFO_CPUID_FAULT bit unconditionally.  This made
guests incorrectly believe the VM emulates
MSR_MISC_FEATURES_ENABLES properly (which is not true because
QEMU has no migration code to handle the MSR).

The KVM_CAP_MSR_PLATFORM_INFO capability was added[2] to Linux
v4.19 to address the issue.  Use it to disable emulation of
MSR_PLATFORM_INFO and stop incorrectly exposing cpuid_fault to
guests.

References:
[1] commit db2336a80489 ("KVM: x86: virtualize cpuid faulting")
[2] commit 6fbbde9a1969 ("KVM: x86: Control guest reads of MSR_PLATFORM_INFO")

Reported-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 target/i386/kvm.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index 9af4542fb8..9629f25c90 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -1647,6 +1647,14 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
         }
     }
 
+    /*
+     * QEMU doesn't initialize MSR_PLATFORM_INFO yet, so disable the MSR
+     * unconditionally until support for the MSR is properly implemented
+     */
+    if (kvm_check_extension(s, KVM_CAP_MSR_PLATFORM_INFO)) {
+        kvm_vm_enable_cap(s, KVM_CAP_MSR_PLATFORM_INFO, 0);
+    }
+
     return 0;
 }
 
-- 
2.18.0.rc1.1.g3f1ff2140