[RFC PATCH 1/1] i386: Remove features from Epyc-Milan cpu

[Leonardo Bras <leobras@redhat.com>]

While trying to bring a VM with EPYC-Milan cpu on a host with
EPYC-Milan cpu (EPYC 7313), the following warning can be seen:

qemu-system-x86_64: warning: host doesn't support requested feature: CPUID.07H:EBX.erms [bit 9]
qemu-system-x86_64: warning: host doesn't support requested feature: CPUID.07H:EDX.fsrm [bit 4]

Even with this warning, the host goes up.

Then, grep'ing cpuid output on both guest and host, outputs:

extended feature flags (7):
      enhanced REP MOVSB/STOSB                 = false
      fast short REP MOV                       = false
      (simple synth)  = AMD EPYC (3rd Gen) (Milan B1) [Zen 3], 7nm
   brand = "AMD EPYC 7313 16-Core Processor               "

This means that for the same -cpu model (EPYC-Milan), the vcpu may or may
not have the above feature bits set, which is usually not a good idea for
live migration:
Migrating from a host with these features to a host without them can
be troublesome for the guest.

Remove the "optional" features (erms, fsrm) from Epyc-Milan, in order to
avoid possible after-migration guest issues.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
---

Does this make sense? Or maybe I am missing something here.

Having a kvm guest running with a feature bit, while the host
does not support it seems to cause a possible break the guest.


 target/i386/cpu.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index aa9e636800..a4bbd38ed0 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -4160,12 +4160,9 @@ static const X86CPUDefinition builtin_x86_defs[] = {
             CPUID_7_0_EBX_FSGSBASE | CPUID_7_0_EBX_BMI1 | CPUID_7_0_EBX_AVX2 |
             CPUID_7_0_EBX_SMEP | CPUID_7_0_EBX_BMI2 | CPUID_7_0_EBX_RDSEED |
             CPUID_7_0_EBX_ADX | CPUID_7_0_EBX_SMAP | CPUID_7_0_EBX_CLFLUSHOPT |
-            CPUID_7_0_EBX_SHA_NI | CPUID_7_0_EBX_CLWB | CPUID_7_0_EBX_ERMS |
-            CPUID_7_0_EBX_INVPCID,
+            CPUID_7_0_EBX_SHA_NI | CPUID_7_0_EBX_CLWB | CPUID_7_0_EBX_INVPCID,
         .features[FEAT_7_0_ECX] =
             CPUID_7_0_ECX_UMIP | CPUID_7_0_ECX_RDPID | CPUID_7_0_ECX_PKU,
-        .features[FEAT_7_0_EDX] =
-            CPUID_7_0_EDX_FSRM,
         .features[FEAT_XSAVE] =
             CPUID_XSAVE_XSAVEOPT | CPUID_XSAVE_XSAVEC |
             CPUID_XSAVE_XGETBV1 | CPUID_XSAVE_XSAVES,
-- 
2.34.1