Re: [PATCH 7/8] KVM: x86/mmu: Alloc TDP MMU roots while holding mmu_lock for read

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 06, 2024, Xu Yilun wrote:
> On Wed, Jan 10, 2024 at 06:00:47PM -0800, Sean Christopherson wrote:
> > ---
> >  arch/x86/kvm/mmu/tdp_mmu.c | 55 +++++++++++++++-----------------------
> >  1 file changed, 22 insertions(+), 33 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 9a8250a14fc1..d078157e62aa 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -223,51 +223,42 @@ static void tdp_mmu_init_child_sp(struct kvm_mmu_page *child_sp,
> >  	tdp_mmu_init_sp(child_sp, iter->sptep, iter->gfn, role);
> >  }
> >  
> > -static struct kvm_mmu_page *kvm_tdp_mmu_try_get_root(struct kvm_vcpu *vcpu)
> > -{
> > -	union kvm_mmu_page_role role = vcpu->arch.mmu->root_role;
> > -	int as_id = kvm_mmu_role_as_id(role);
> > -	struct kvm *kvm = vcpu->kvm;
> > -	struct kvm_mmu_page *root;
> > -
> > -	for_each_valid_tdp_mmu_root_yield_safe(kvm, root, as_id) {
> > -		if (root->role.word == role.word)
> > -			return root;
> > -	}
> > -
> > -	return NULL;
> > -}
> > -
> >  int kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu)
> >  {
> >  	struct kvm_mmu *mmu = vcpu->arch.mmu;
> >  	union kvm_mmu_page_role role = mmu->root_role;
> > +	int as_id = kvm_mmu_role_as_id(role);
> >  	struct kvm *kvm = vcpu->kvm;
> >  	struct kvm_mmu_page *root;
> >  
> >  	/*
> > -	 * Check for an existing root while holding mmu_lock for read to avoid
> > +	 * Check for an existing root before acquiring the pages lock to avoid
> >  	 * unnecessary serialization if multiple vCPUs are loading a new root.
> >  	 * E.g. when bringing up secondary vCPUs, KVM will already have created
> >  	 * a valid root on behalf of the primary vCPU.
> >  	 */
> >  	read_lock(&kvm->mmu_lock);
> > -	root = kvm_tdp_mmu_try_get_root(vcpu);
> > -	read_unlock(&kvm->mmu_lock);
> >  
> > -	if (root)
> > -		goto out;
> > +	for_each_valid_tdp_mmu_root_yield_safe(kvm, root, as_id) {
> > +		if (root->role.word == role.word)
> > +			goto out_read_unlock;
> > +	}
> >  
> > -	write_lock(&kvm->mmu_lock);
> 
> It seems really complex to me...
> 
> I failed to understand why the following KVM_BUG_ON() could be avoided
> without the mmu_lock for write. I thought a valid root could be added
> during zapping.
> 
>   void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
>   {
> 	struct kvm_mmu_page *root;
> 
> 	read_lock(&kvm->mmu_lock);
> 
> 	for_each_tdp_mmu_root_yield_safe(kvm, root) {
> 		if (!root->tdp_mmu_scheduled_root_to_zap)
> 			continue;
> 
> 		root->tdp_mmu_scheduled_root_to_zap = false;
> 		KVM_BUG_ON(!root->role.invalid, kvm);

tdp_mmu_scheduled_root_to_zap is set only when mmu_lock is held for write, i.e.
it's mutually exclusive with allocating a new root.

And tdp_mmu_scheduled_root_to_zap is cleared if and only if kvm_tdp_mmu_zap_invalidated_roots
is already set, and is only processed by kvm_tdp_mmu_zap_invalidated_roots(),
which runs under slots_lock (a mutex).

So a new, valid root can be added, but it won't have tdp_mmu_scheduled_root_to_zap
set, at least not until the current "fast zap" completes and a new one beings,
which as above requires taking mmu_lock for write.