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

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Message-ID: <20201103062718.GD12408@shuo-intel.sh.intel.com>
Date:   Tue, 3 Nov 2020 14:27:18 +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

Hi Boris,

On Mon  2.Nov'20 at 15:37:07 +0100, Borislav Petkov wrote:
>On Mon, Oct 19, 2020 at 02:17:49PM +0800, shuo.a.liu@...el.com wrote:
>> +bool acrn_is_privileged_vm(void)
>> +{
>> +	return cpuid_eax(acrn_cpuid_base() | ACRN_CPUID_FEATURES) &
>> +			 ACRN_FEATURE_PRIVILEGED_VM;
>
>I asked in the previous review why that acrn_cpuid_base() is used here,
>you said that the base might vary. Looking at hypervisor_cpuid_base(),
>it searches in the range [0x40000000, 0x40010000] with an 0x100 offset.
>
>So you're saying that ACRN_CPUID_FEATURES is the first leaf beyond the
>base. Close?

Yes.

>
>If so, why isn't the code doing this?
>
>	return cpuid_eax(acrn_cpuid_base() + 1)...
>
>and why doesn't it have a comment above it explaining that the base can
>change and it needs to be discovered each time?

The code just followed KVM style (see kvm_arch_para_features()).
I can change to use
	cpuid_eax(acrn_cpuid_base() + 1)...
If you prefer to.

hypervisor_cpuid_base() implies the base is variable, no? We use
this function to detect the base.

>
>> +EXPORT_SYMBOL_GPL(acrn_is_privileged_vm);
>
>Also, that acrn_is_privileged_vm() silly helper is used only once and
>I don't like the exported symbols pollution we're doing. So make that
>function give you the eax of ACRN_CPUID_FEATURES and callers can do
>their testing themselves.

OK. Then i will define acrn_cpuid_base() as a static inline function in
asm/acrn.h for callers.

>
>When it turns out that code patterns get repeated, you can then
>aggregate stuff into a helper.

Got it. Thanks.

Thanks
shuo