On Tue, Feb 06, 2024 at 10:10:44AM -0800, Sean Christopherson wrote:
> 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.
It's clear to me.
Thanks for the detailed explanation.
>
