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

From: Like Xu
Date: Wed Jun 05 2019 - 21:34:55 EST

On 2019/6/4 3:18, Sean Christopherson wrote:
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.

Hi Radim & Sean,

Thanks for your review and finally we find a better way.

Please help review the new version, I am not sure that the changelog could help new submitter understand why the clamping exists.