Re: [RESEND PATCH v3] KVM: x86: Add Intel CPUID.1F cpuid emulation support

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Jun 03, 2019 at 06:56:17PM +0200, Radim Krčmář wrote:
> > +			break;
> > +		}
> >  		entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
> 
> Similarly in the existing code.  If we don't have f_intel_pt, then we
> should make sure that leaf 0x14 is not being filled, but we don't really
> have to limit the maximal index.
> 
> Adding a single clamping like
> 
> 		/* Limited to the highest leaf implemented in KVM. */
> 		entry->eax = min(entry->eax, 0x1f);
> 
> seems sufficient.
>
> (Passing the hardware value is ok in theory, but it is a cheap way to
>  avoid future leaves that cannot be simply zeroed for some weird reason.)

I don't have a strong opinion regarding the code itself, but whatever ends
up getting committed should have a big beefy changelog explaining why the
clamping exists, or at least extolling its virtues.  I had a hell of a
time understanding the intent of this one line of code because as your
response shows, there is no one right answer.