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

From: Sean Christopherson
Date: Mon Jun 03 2019 - 15:22:11 EST

On Mon, Jun 03, 2019 at 06:56:17PM +0200, Radim KrÄmÃÅ 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.