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

[Sean Christopherson <seanjc@google.com>]

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@google.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.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.

> >  		    kvm_tdp_mmu_get_root(next_root))
> >  			break;
> >