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SubjectRe: [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@intel.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

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