Re: [PATCH 04/22] kvm: mmu: Allocate and free TDP MMU roots

[Sean Christopherson <sean.j.christopherson@xxxxxxxxx>]

On Wed, Sep 30, 2020 at 08:26:28AM +0200, Paolo Bonzini wrote:
> On 30/09/20 08:06, Sean Christopherson wrote:
> >> +static struct kvm_mmu_page *get_tdp_mmu_vcpu_root(struct kvm_vcpu *vcpu)
> >> +{
> >> +	struct kvm_mmu_page *root;
> >> +	union kvm_mmu_page_role role;
> >> +
> >> +	role = vcpu->arch.mmu->mmu_role.base;
> >> +	role.level = vcpu->arch.mmu->shadow_root_level;
> >> +	role.direct = true;
> >> +	role.gpte_is_8_bytes = true;
> >> +	role.access = ACC_ALL;
> >> +
> >> +	spin_lock(&vcpu->kvm->mmu_lock);
> >> +
> >> +	/* Search for an already allocated root with the same role. */
> >> +	root = find_tdp_mmu_root_with_role(vcpu->kvm, role);
> >> +	if (root) {
> >> +		get_tdp_mmu_root(vcpu->kvm, root);
> >> +		spin_unlock(&vcpu->kvm->mmu_lock);
> > Rather than manually unlock and return, this can be
> > 
> > 	if (root)
> > 		get_tdp_mmju_root();
> > 
> > 	spin_unlock()
> > 
> > 	if (!root)
> > 		root = alloc_tdp_mmu_root();
> > 
> > 	return root;
> > 
> > You could also add a helper to do the "get" along with the "find".  Not sure
> > if that's worth the code.
> 
> All in all I don't think it's any clearer than Ben's code.  At least in
> his case the "if"s clearly point at the double-checked locking pattern.

Actually, why is this even dropping the lock to do the alloc?  The allocs are
coming from the caches, which are designed to be invoked while holding the
spin lock.

Also relevant is that, other than this code, the only user of
find_tdp_mmu_root_with_role() is kvm_tdp_mmu_root_hpa_for_role(), and that
helper is itself unused.  I.e. the "find" can be open coded.

Putting those two together yields this, which IMO is much cleaner.

static struct kvm_mmu_page *get_tdp_mmu_vcpu_root(struct kvm_vcpu *vcpu)
{
        union kvm_mmu_page_role role;
	struct kvm *kvm = vcpu->kvm;
        struct kvm_mmu_page *root;

	role = page_role_for_level(vcpu, vcpu->arch.mmu->shadow_root_level);

        spin_lock(&kvm->mmu_lock);

        /* Check for an existing root before allocating a new one. */
        for_each_tdp_mmu_root(kvm, root) {
                if (root->role.word == role.word) {
                        get_tdp_mmu_root(root);
                        spin_unlock(&kvm->mmu_lock);
                        return root;
                }
        }

        root = alloc_tdp_mmu_page(vcpu, 0, vcpu->arch.mmu->shadow_root_level);
        root->root_count = 1;

        list_add(&root->link, &kvm->arch.tdp_mmu_roots);

        spin_unlock(&kvm->mmu_lock);

        return root;
}