linux-kernel - Re: [PATCH v5 03/17] x86/acrn: Introduce an API to check if a VM is privileged

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Date:   Thu, 5 Nov 2020 11:25:22 +0800
From:   Shuo A Liu <shuo.a.liu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yu Wang <yu1.wang@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>
Subject: Re: [PATCH v5 03/17] x86/acrn: Introduce an API to check if a VM is
 privileged

On Wed  4.Nov'20 at 19:51:57 +0100, Borislav Petkov wrote:
>On Wed, Nov 04, 2020 at 11:50:27AM +0800, Shuo A Liu wrote:
>> On Tue  3.Nov'20 at 11:25:38 +0100, Borislav Petkov wrote:
>> > On Tue, Nov 03, 2020 at 02:27:18PM +0800, Shuo A Liu wrote:
>> > > The code just followed KVM style (see kvm_arch_para_features()).
>> >
>> > Do you see Documentation/virt/kvm/cpuid.rst?
>>
>> OK. It documents the leaf number.
>
>It also says
>
>"Note also that old hosts set eax value to 0x0. This should be
>interpreted as if the value was 0x40000001."

Ok.

>
>Does this hold true for the acrn HV? The fact that I'm asking about
>all those things should give you a hint that documenting the API is
>important.

No, acrn HV is always return eax with maximum leaf number.

Let me add the document.

>
>> > Now where is yours explaining what your hypervisor is doing?
>>
>> Currently, it is in arch/x86/include/asm/acrn.h.
>
>Yeah, you can't expect people to go scrape it from headers though - it
>should be concentrated in a doc file.
>
>> If the leaf numbers be documented explicitly (like kvm), i think i can
>> use them as eax of cpuid_eax() directly (back to your first comment).
>
>Which means, you don't need to do the logical OR-ing which kvm does
>because of what I pasted above about eax being 0 on old hosts. Now we're
>getting somewhere...

Yes.

>
>> 	cpuid_eax(ACRN_CPUID_FEATURES)...
>>
>> If you looking at implementation of acrn-hypervisor, you will found the
>> leaf number is hardcoded in the hypervisor. So, they also can be
>> documented explicitly.
>
>Ok.
>
>> OK. I can add a similar cpuid.rst for acrn.
>
>Yes please.
>
>> Yes. Fix patches are always welcome.
>
>Ok, good, the thing is open. You could put that in the doc too, along
>with the link so that people can find it.

OK.

Thanks
shuo