Re: [PATCH v2 02/15] mshyperv: Introduce hv_get_hypervisor_version function

[Dave Hansen]

On 8/17/23 15:01, Nuno Das Neves wrote:
> +int hv_get_hypervisor_version(union hv_hypervisor_version_info *info)
> +{
> +	hv_get_vpreg_128(HV_REGISTER_HYPERVISOR_VERSION,
> +			 (struct hv_get_vp_registers_output *)info);
> +
> +	return 0;
> +}
...
> +int hv_get_hypervisor_version(union hv_hypervisor_version_info *info)
> +{
> +	unsigned int hv_max_functions;
> +
> +	hv_max_functions = cpuid_eax(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS);
> +	if (hv_max_functions < HYPERV_CPUID_VERSION) {
> +		pr_err("%s: Could not detect Hyper-V version\n",
> +			__func__);
> +		return -ENODEV;
> +	}
> +
> +	info->eax = cpuid_eax(HYPERV_CPUID_VERSION);
> +	info->ebx = cpuid_ebx(HYPERV_CPUID_VERSION);
> +	info->ecx = cpuid_ecx(HYPERV_CPUID_VERSION);
> +	info->edx = cpuid_edx(HYPERV_CPUID_VERSION);
> +
> +	return 0;
> +}

I can't help but notice that the ARM version does *one* call to the
hardware while the x86 version does CPUID four different times, once to
get *EACH* register and throwing away the other three registers each
time.  Also recall that CPUID is a big, fat architecturally serializing
instruction.  This isn't a fast path, but CPUID is about as slow as you get.

Is there any reason you can't just have an x86 version of
hv_get_vpreg_128() that gets the 128 bits bytes of data that comes back
in the 4 CPUID registers?

That might even let you share some more code.