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

[Shuo A Liu]

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@xxxxxxxxx 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