linux-kernel - Re: [PATCH 1/7] hyperv: Detect Nested virtualization support for SVM

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Message-ID: <af87c25e-78c6-5859-e1c1-2aa07d087a25@linux.microsoft.com>
Date:   Thu, 8 Apr 2021 09:15:47 -0400
From:   Vineeth Pillai <viremana@...ux.microsoft.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "K. Y. Srinivasan" <kys@...rosoft.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org,
        Lan Tianyu <Tianyu.Lan@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Wei Liu <wei.liu@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>
Subject: Re: [PATCH 1/7] hyperv: Detect Nested virtualization support for SVM

Hi Vitaly,

On 4/8/21 7:06 AM, Vitaly Kuznetsov wrote:
> -	if (ms_hyperv.hints & HV_X64_ENLIGHTENED_VMCS_RECOMMENDED) {
> +	/*
> +	 * AMD does not need enlightened VMCS as VMCB is already a
> +	 * datastructure in memory.
> Well, VMCS is also a structure in memory, isn't it? It's just that we
> don't have a 'clean field' concept for it and we can't use normal memory
> accesses.

Yes, you are right. I was referring to the fact that we cant use normal
memory accesses, but is a bit mis-worded.

>
>> 	We need to get the nested
>> +	 * features if SVM is enabled.
>> +	 */
>> +	if (boot_cpu_has(X86_FEATURE_SVM) ||
>> +	    ms_hyperv.hints & HV_X64_ENLIGHTENED_VMCS_RECOMMENDED) {
> Do I understand correctly that we can just look at CPUID.0x40000000.EAX
> and in case it is >= 0x4000000A we can read HYPERV_CPUID_NESTED_FEATURES
> leaf? I'd suggest we do that intead then.
I agree, that is a better way to consolidate both the cases.
Will change it in the next iteration. Probably the above code
comment is not needed when we consolidate the cases here.

Thanks,
Vineeth