linux-kernel - Re: [PATCH 21/28] KVM: x86/mmu: Add TDP MMU helper to zap a root

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Message-ID: <YZwkpcmAi07fOgbz@google.com>
Date:   Mon, 22 Nov 2021 23:15:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hou Wenlong <houwenlong93@...ux.alibaba.com>
Subject: Re: [PATCH 21/28] KVM: x86/mmu: Add TDP MMU helper to zap a root

On Mon, Nov 22, 2021, Ben Gardon wrote:
> On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > Add a small wrapper to handle zapping a specific root.  For now, it's
> > little more than syntactic sugar, but in the future it will become a
> > unique flow with rules specific to zapping an unreachable root.
> >
> > No functional change intended.
> >
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kvm/mmu/tdp_mmu.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 9449cb5baf0b..31fb622249e5 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -79,11 +79,18 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
> >         tdp_mmu_free_sp(sp);
> >  }
> >
> > +static bool tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> > +                            bool shared)
> > +{
> > +       return zap_gfn_range(kvm, root, 0, -1ull, true, false, shared);
> 
> Total aside:
> Remembering the order of these three boolean parameters through all
> these functions drives me nuts.
> It'd be really nice to put them into a neat, reusable struct that tracks:
> MMU lock mode (read / write / none)
> If yielding is okay
> If the TLBs are dirty and need to be flushed
> 
> I don't know when I'll have time to do that refactor, but it would
> make this code so much more sensible.

Heh, I did exactly that, then threw away the code when I realized that I could
break up zap_gfn_range() into three separate helpers and avoid control knob hell
(spoiler alert for later patches in this series).

There are still two booleans (to what ends up being tdp_mmu_zap_leafs()), but none
none of the call sites pass true/false for _both_ params, so the call sites end up
being quite readable.  At that point, using a struct ended up being a net negative,
e.g. kvm_tdp_mmu_unmap_gfn_range() had to marshall from one struct to another.