i386: kvm: Disable arch_capabilities if MSR can't be set

KVM has two bugs in the handling of MSR_IA32_ARCH_CAPABILITIES:

1) Linux commit commit 1eaafe91a0df ("kvm: x86: IA32_ARCH_CAPABILITIES
   is always supported") makes GET_SUPPORTED_CPUID return
   arch_capabilities even if running on SVM.  This makes "-cpu
   host,migratable=off" incorrectly expose arch_capabilities on CPUID on
   AMD hosts (where the MSR is not emulated by KVM).

2) KVM_GET_MSR_INDEX_LIST does not return MSR_IA32_ARCH_CAPABILITIES if
   the MSR is not supported by the host CPU.  This makes QEMU not
   initialize the MSR properly at kvm_put_msrs() on those hosts.

Work around both bugs on the QEMU side, by checking if the MSR
was returned by KVM_GET_MSR_INDEX_LIST before returning the
feature flag on kvm_arch_get_supported_cpuid().

This has the unfortunate side effect of making arch_capabilities
unavailable on hosts without hardware support for the MSR until bug #2
is fixed on KVM, but I can't see another way to work around bug xen-project#1
without that side effect.

Signed-off-by: Eduardo Habkost <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>

target/i386/kvm.c

@@ -389,6 +389,15 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
         if (host_tsx_blacklisted()) {
             ret &= ~(CPUID_7_0_EBX_RTM | CPUID_7_0_EBX_HLE);
         }
+    } else if (function == 7 && index == 0 && reg == R_EDX) {
+        /*
+         * Linux v4.17-v4.20 incorrectly return ARCH_CAPABILITIES on SVM hosts.
+         * We can detect the bug by checking if MSR_IA32_ARCH_CAPABILITIES is
+         * returned by KVM_GET_MSR_INDEX_LIST.
+         */
+        if (!has_msr_arch_capabs) {
+            ret &= ~CPUID_7_0_EDX_ARCH_CAPABILITIES;
+        }
     } else if (function == 0x80000001 && reg == R_ECX) {
         /*
          * It's safe to enable TOPOEXT even if it's not returned by