linux-kernel - Re: [PATCH v3 01/28] KVM: x86/mmu: Use common iterator for walking invalid TDP MMU roots

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Date:   Thu, 3 Mar 2022 00:57:20 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3 01/28] KVM: x86/mmu: Use common iterator for walking
 invalid TDP MMU roots

On Wed, Mar 02, 2022, Sean Christopherson wrote:
> On Wed, Mar 02, 2022, Mingwei Zhang wrote:
> > On Sat, Feb 26, 2022, Sean Christopherson wrote:
> > > Now that tdp_mmu_next_root() can process both valid and invalid roots,
> > > extend it to be able to process _only_ invalid roots, add yet another
> > > iterator macro for walking invalid roots, and use the new macro in
> > > kvm_tdp_mmu_zap_invalidated_roots().
> > > 
> > > No functional change intended.
> > > 
> > > Reviewed-by: David Matlack <dmatlack@...gle.com>
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > ---
> > >  arch/x86/kvm/mmu/tdp_mmu.c | 74 ++++++++++++++------------------------
> > >  1 file changed, 26 insertions(+), 48 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > > index debf08212f12..25148e8b711d 100644
> > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > @@ -98,6 +98,12 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
> > >  	call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
> > >  }
> > >  
> > > +enum tdp_mmu_roots_iter_type {
> > > +	ALL_ROOTS = -1,
> > > +	VALID_ROOTS = 0,
> > > +	INVALID_ROOTS = 1,
> > > +};
> > 
> > I am wondering what the trick is to start from -1?
> 
> -1 is arbitrary, any non-zero value would work.  More below.
> 
> > >  /*
> > >   * Returns the next root after @prev_root (or the first root if @prev_root is
> > >   * NULL).  A reference to the returned root is acquired, and the reference to
> > > @@ -110,10 +116,16 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
> > >   */
> > >  static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> > >  					      struct kvm_mmu_page *prev_root,
> > > -					      bool shared, bool only_valid)
> > > +					      bool shared,
> > > +					      enum tdp_mmu_roots_iter_type type)
> > >  {
> > >  	struct kvm_mmu_page *next_root;
> > >  
> > > +	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
> > > +
> > > +	/* Ensure correctness for the below comparison against role.invalid. */
> > > +	BUILD_BUG_ON(!!VALID_ROOTS || !INVALID_ROOTS);
> > > +
> > >  	rcu_read_lock();
> > >  
> > >  	if (prev_root)
> > > @@ -125,7 +137,7 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> > >  						   typeof(*next_root), link);
> > >  
> > >  	while (next_root) {
> > > -		if ((!only_valid || !next_root->role.invalid) &&
> > > +		if ((type == ALL_ROOTS || (type == !!next_root->role.invalid)) &&
> 
> This is the code that deals with the enums.  It's making the type a tri-state,
> where the values of VALID_ROOTS and INVALID_ROOTS align with converting role.invalid
> to a boolean (always '0' or '1') so that they can be directly compared as above.
> 
> Any value for ALL_ROOTS (other than '0' or '1' obviously) would work since the
> above logic requires ALL_ROOTS to be explicitly checked first.
> 
yeah, I see that. The other thing I feel strange is the that VALID_ROOTS
is _0_ while INVALID_ROOTS is _1_. But when I see !!next_root->role.invalid,
that solves my concerns.
> > >  		    kvm_tdp_mmu_get_root(next_root))
> > >  			break;
> > >