[Qemu-devel] [PATCH 1/2] i386: kvm: Disable arch_capabilities if MSR can't be set

[Eduardo Habkost <ehabkost@redhat.com>]

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 #1
without that side effect.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: KarimAllah Ahmed <karahmed@amazon.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Darren Kenny <darren.kenny@oracle.com>
---
 target/i386/kvm.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index 9af4542fb8..4fa3e3806a 100644
--- a/target/i386/kvm.c
+++ b/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 incorrectly v4.17-v4.20 return ARCH_CAPABILITIES on SVM.
+         * 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
-- 
2.18.0.rc1.1.g3f1ff2140