linux-kernel - Re: [PATCH v2 02/15] mshyperv: Introduce hv_get_hypervisor

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Date:   Thu, 17 Aug 2023 16:43:49 -0700
From:   Nuno Das Neves <nunodasneves@...ux.microsoft.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org
Cc:     patches@...ts.linux.dev, mikelley@...rosoft.com, kys@...rosoft.com,
        wei.liu@...nel.org, haiyangz@...rosoft.com, decui@...rosoft.com,
        apais@...ux.microsoft.com, Tianyu.Lan@...rosoft.com,
        ssengar@...ux.microsoft.com, mukeshrathor@...rosoft.com,
        stanislav.kinsburskiy@...il.com, jinankjain@...ux.microsoft.com,
        vkuznets@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        will@...nel.org, catalin.marinas@....com
Subject: Re: [PATCH v2 02/15] mshyperv: Introduce hv_get_hypervisor_version
 function

On 8/17/2023 4:14 PM, Dave Hansen wrote:
> 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.
> 

Good point, it is quite wasteful. I just went and checked cpuid.h, and of
course there is a generic cpuid() function that can get all 4 at once.
I will update it to use this in the next version. Thanks!

> 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.

Unfortunately, for some reason the hypervisor only lets you get this data via
CPUID on x86_64, and via hypercall - i.e. hv_get_vpreg_128() - on ARM64.