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

[Daniel P . Berrangé]

CC'ing  Babu Moger who aded the Milan CPU model.

On Sat, Jan 29, 2022 at 07:23:37AM -0300, Leonardo Bras wrote:
> 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.

Babu,  can you give some insight into availability of erms / fsrm
features across the EPYC 3rd gen CPU line. Is this example missing
erms/fsrm an exception, or common place ?

> 
> 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.

The guest won't see the feature bit - that warning message from QEMU
is telling you that it did't honour the request to expose
erms / fsrm - it has dropped them from the CPUO exposed to 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
> 
> 

Regards,
Daniel
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