From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>

Since KVM commit 5f76f6f5ff96 ("KVM: nVMX: Do not expose MPX VMX controls when guest MPX disabled")
it is not possible to disable MPX on a "-cpu host" just by adding "-mpx"
there if the host CPU does indeed support MPX.
QEMU will fail to set MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS MSRs in this case
and so trigger an assertion failure.

Instead, besides "-mpx" one has to explicitly add also
"-vmx-exit-clear-bndcfgs" and "-vmx-entry-load-bndcfgs" to QEMU command
line to make it work, which is a bit convoluted.

Make the MPX-related bits in FEAT_VMX_{EXIT,ENTRY}_CTLS dependent on MPX
being actually enabled so such workarounds are no longer necessary.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
---

Changes from v1:
    Use feature_dependencies instead of sanitizing MPX-related bits in
    MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS when setting them.

 target/i386/cpu.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 35c3475e6c..385691458f 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -1355,6 +1355,14 @@ static FeatureDep feature_dependencies[] = {
         .from = { FEAT_7_0_EBX,             CPUID_7_0_EBX_INVPCID },
         .to = { FEAT_VMX_SECONDARY_CTLS,    VMX_SECONDARY_EXEC_ENABLE_INVPCID },
     },
+    {
+        .from = { FEAT_7_0_EBX,             CPUID_7_0_EBX_MPX },
+        .to = { FEAT_VMX_EXIT_CTLS,         VMX_VM_EXIT_CLEAR_BNDCFGS },
+    },
+    {
+        .from = { FEAT_7_0_EBX,             CPUID_7_0_EBX_MPX },
+        .to = { FEAT_VMX_ENTRY_CTLS,        VMX_VM_ENTRY_LOAD_BNDCFGS },
+    },
     {
         .from = { FEAT_7_0_EBX,             CPUID_7_0_EBX_RDSEED },
         .to = { FEAT_VMX_SECONDARY_CTLS,    VMX_SECONDARY_EXEC_RDSEED_EXITING },

Queued, thanks.

Paolo