Re: [Qemu-devel] [PATCH v3 6/9] i386/cpu: Add CPUID.1F generation support for multi-dies PCMachine

[Eduardo Habkost <ehabkost@redhat.com>]

I've just noticed one thing I don't understand:

On Wed, Jun 12, 2019 at 04:41:01PM +0800, Like Xu wrote:
> The CPUID.1F as Intel V2 Extended Topology Enumeration Leaf would be
> exposed if guests want to emulate multiple software-visible die within
> each package. Per Intel's SDM, the 0x1f is a superset of 0xb, thus they
> can be generated by almost same code as 0xb except die_offset setting.
> 
> If the number of dies per package is less than 2, the qemu will not
> expose CPUID.1F regardless of whether the host supports CPUID.1F.
> 
> Signed-off-by: Like Xu <like.xu@linux.intel.com>
> ---
>  target/i386/cpu.c | 37 +++++++++++++++++++++++++++++++++++++
>  target/i386/cpu.h |  4 ++++
>  target/i386/kvm.c | 12 ++++++++++++
>  3 files changed, 53 insertions(+)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 09e20a2c3b..127aff74a6 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -4437,6 +4437,42 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
>              *ecx |= CPUID_TOPOLOGY_LEVEL_INVALID;
>          }
>  
> +        assert(!(*eax & ~0x1f));
> +        *ebx &= 0xffff; /* The count doesn't need to be reliable. */
> +        break;
> +    case 0x1F:
> +        /* V2 Extended Topology Enumeration Leaf */
> +        if (env->nr_dies < 2 || !cpu->enable_cpuid_0x1f) {
> +            *eax = *ebx = *ecx = *edx = 0;

Why exactly do you need cpu->enable_cpuid_0x1f?  When would it
make sense to set dies > 1 but disable CPUID.1F?


> +            break;
> +        }
[...]

-- 
Eduardo