linux-kernel - Re: [PATCH v2 5/9] KVM: x86/mmu: Factor out the meat of reset_tdp_shadow_zero_bits

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Message-ID: <CANgfPd8a3opGRKqzgWj9bAUx42wXG9Wc2HrsgtP6PoTBttQ3+w@mail.gmail.com>
Date:   Thu, 21 Apr 2022 11:50:37 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH v2 5/9] KVM: x86/mmu: Factor out the meat of reset_tdp_shadow_zero_bits_mask

On Tue, Apr 12, 2022 at 8:46 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Mon, Mar 21, 2022, Ben Gardon wrote:
> > Factor out the implementation of reset_tdp_shadow_zero_bits_mask to a
> > helper function which does not require a vCPU pointer. The only element
> > of the struct kvm_mmu context used by the function is the shadow root
> > level, so pass that in too instead of the mmu context.
> >
> > No functional change intended.
> >
> > Signed-off-by: Ben Gardon <bgardon@...gle.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 17 +++++++++++------
> >  1 file changed, 11 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 3b8da8b0745e..6f98111f8f8b 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4487,16 +4487,14 @@ static inline bool boot_cpu_is_amd(void)
> >   * possible, however, kvm currently does not do execution-protection.
> >   */
> >  static void
>
> Strongly prefer the newline here get dropped (see below).
>
> > -reset_tdp_shadow_zero_bits_mask(struct kvm_mmu *context)
> > +build_tdp_shadow_zero_bits_mask(struct rsvd_bits_validate *shadow_zero_check,
>
> Kind of a nit, but KVM uses "calc" for this sort of thing.  There are no other
> instances of "build_" to describe this behavior.
>
> Am I alone in think that shadow_zero_check is an awful, awful name?  E.g. the EPT
> memtype case has legal non-zero values.  Anyone object to opportunistically
> renaming the function and the local shadow_zero_check to "rsvd_bits" to shorten
> line lengths and move KVM one step closer to consistent naming?

That makes sense to me. I'm happy to add a commit to this series to
standardize on rsvd_bits.

>
> > +                             int shadow_root_level)
> >  {
> > -     struct rsvd_bits_validate *shadow_zero_check;
> >       int i;
> >
> > -     shadow_zero_check = &context->shadow_zero_check;
> > -
> >       if (boot_cpu_is_amd())
> >               __reset_rsvds_bits_mask(shadow_zero_check, reserved_hpa_bits(),
> > -                                     context->shadow_root_level, false,
> > +                                     shadow_root_level, false,
> >                                       boot_cpu_has(X86_FEATURE_GBPAGES),
> >                                       false, true);
> >       else
> > @@ -4507,12 +4505,19 @@ reset_tdp_shadow_zero_bits_mask(struct kvm_mmu *context)
> >       if (!shadow_me_mask)
> >               return;
> >
> > -     for (i = context->shadow_root_level; --i >= 0;) {
> > +     for (i = shadow_root_level; --i >= 0;) {
> >               shadow_zero_check->rsvd_bits_mask[0][i] &= ~shadow_me_mask;
> >               shadow_zero_check->rsvd_bits_mask[1][i] &= ~shadow_me_mask;
> >       }
> >  }
> >
> > +static void
> > +reset_tdp_shadow_zero_bits_mask(struct kvm_mmu *context)
>
> One line!  Aside from being against the One True Style[*], there is zero reason
> for a newline here.
>
> And I vote to drop the "mask", because (a) it's not a singular mask and (b) it's
> not even a mask in all cases.
>
> And while I'm on a naming consistency rant, s/context/mmu.
>
> I.e. end up with:
>
> static void calc_tdp_shadow_rsvd_bits(struct rsvd_bits_validate *rsvd_bits,
>                                       int shadow_root_level)
>
> static void reset_tdp_shadow_rsvd_bits(struct kvm_mmu *mmu)
>
> [*] https://lore.kernel.org/mm-commits/CAHk-=wjS-Jg7sGMwUPpDsjv392nDOOs0CtUtVkp=S6Q7JzFJRw@mail.gmail.com
>
> > +{
> > +     build_tdp_shadow_zero_bits_mask(&context->shadow_zero_check,
> > +                                     context->shadow_root_level);
> > +}
> > +
> >  /*
> >   * as the comments in reset_shadow_zero_bits_mask() except it
> >   * is the shadow page table for intel nested guest.
> > --
> > 2.35.1.894.gb6a874cedc-goog
> >