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

Sean Christopherson
2022-03-02T19:51:49Z

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;
> >