What were you trying to do that didn't work?
Currently on Milan, SNP worked cpu models are EPYC-v4 and EPYC-Milan-v2. Besides, CPU host model is blocked by issue RHEL-50060. Hence, on Milan there's no libvirt supported CPU model to boot SNP guest.

Please provide the package NVR for which bug is seen:
qemu-kvm-9.0.0-8.el9.x86_64

How reproducible:
100%

Steps to reproduce
1. Boot SNP guest on Milan with libvirt with cpu model EPYC-Milan-v2
...
<cpu mode='custom' match='exact' check='partial'>
<model fallback='allow'>EPYC-Milan-v2</model>
</cpu>

<launchSecurity type='sev-snp'>
<cbitpos>51</cbitpos>
<reducedPhysBits>1</reducedPhysBits>
<policy>0x00030000</policy>
</launchSecurity>
...
returns error: internal error: process exited while connecting to monitor: 2024-08-20T01:50:10.194740Z qemu-kvm: -accel kvm: vm-type SEV-SNP not supported by KVM

2. Boot SNP guest with qemu -cpu EPYC-Milan and EPYC-IBPB, get
qemu-kvm: SNP_LAUNCH_UPDATE ret=-5 fw_error=22 'Invalid parameter'
qemu-kvm: SEV-SNP: CPUID validation failed for function 0x8000001d, index: 0x3, provided: eax:0x0000c163, ebx: 0x03c0003f, ecx: 0x00007fff, edx: 0x00000006, expected: eax:0x0000c163, ebx: 0x03c0003f, ecx: 0x00007fff, edx: 0x00000002
qemu-kvm: SEV-SNP: failed update CPUID page

Expected results
On Milan, SNP can boot by libvirt

Actual results
No SNP supported CPU model can boot on Milan