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 <leob...@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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