Re: [PATCH 2/3] KVM: x86/mmu: Fix pf_fixed count in tdp_mmu_map_handle_target_level()

[Sean Christopherson <seanjc@google.com>]

On Thu, May 06, 2021, Kai Huang wrote:
> On Wed, 2021-05-05 at 09:11 -0700, Ben Gardon wrote:
> > It would probably also be worth putting a comment on pf_fixed so that
> > people in the future know what it's supposed to mean and we don't get
> > into archeology, reverse engineering the meaning of the stat again.
> 
> It seems the legacy MMU code path is a better place to add the comment to explain when
> pf_fixed should be increased.  However I am not sure whether it is necessary for this
> patch (and I confess I found it's hard to explain why to increase pf_fixed in case of
> emulation :)).  Or perhaps Sean can write a patch to add comment to legacy MMU :)

Ya, I think it makes sense to hold off on documenting the existing behavior in
the TDP MMU.  As is often the case in KVM, just because KVM has always done
something one way, doesn't mean it's correct/ideal.  But, bikeshedding over what
faults exactly should count towards pf_fixed is best left to a separate patch.

> I ended up with  below, by adding a comment in TDP MMU saying "to make it consistent with
> legacy MMU...", and in the commit message, I put a lore link of this discussion, since I
> found Sean's explanation is quite useful. When people are interested in, they can do a git
> blame and find the commit msg of this change -- although it is not as straightforward as
> having comment directly.
> 
> Is this OK to you?
> 
> And Sean?

Yep, works for me.