Re: [Qemu-devel] [PATCH 2/2] x86: hv_evmcs CPU flag support

[Roman Kagan]

On Fri, Oct 19, 2018 at 01:14:32PM +0200, Vitaly Kuznetsov wrote:
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -798,6 +798,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
>      uint32_t unused;
>      struct kvm_cpuid_entry2 *c;
>      uint32_t signature[3];
> +    uint16_t evmcs_version;
>      int kvm_base = KVM_CPUID_SIGNATURE;
>      int r;
>      Error *local_err = NULL;
> @@ -841,7 +842,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
>              memset(signature, 0, 12);
>              memcpy(signature, cpu->hyperv_vendor_id, len);
>          }
> -        c->eax = HV_CPUID_MIN;
> +        c->eax = cpu->hyperv_evmcs ? HV_CPUID_MIN_NESTED : HV_CPUID_MIN;


I think these two aren't meant to be used on the hypervisor side.  My
understanding is that HV_CPUID_MIN is only there as a reminder that the
real Hyper-V exposes at least that many hypervisor-specific leaves so
the guest can rely on that.  So I'd rather use directly
HV_CPUID_IMPLEMENT_LIMITS : HV_CPUID_NESTED_FEATURES, and not introduce
HV_CPUID_MIN_NESTED.  Maybe better yet is to update this field with the
maximum value while populating HV_* leaves:

    if (hyperv_enabled(cpu)) {
        uint32_t *cpuid_40000000_eax;
        c = &cpuid_data.entries[cpuid_i++];
        c->function = HV_CPUID_VENDOR_AND_MAX_FUNCTIONS;
        cpuid_40000000_eax = &c->eax;
        *cpuid_40000000_eax = c->function;

        ....

        c = &cpuid_data.entries[cpuid_i++];
        c->function = HV_CPUID_...;
        *cpuid_40000000_eax = max(*cpuid_40000000_eax, c->function);

but I think it can be done later and doesn't need to hold this patch.

Another question related to this: are the guests OK with leaves
0x40000006..0x40000009 missing?

Thanks,
Roman.