Re: [PATCH v2 11/15] KVM: x86/tdp_mmu: Reflect tearing down mirror page tables

[Edgecombe, Rick P]

On Fri, 2024-06-07 at 13:37 +0200, Paolo Bonzini wrote:
> > 
> > +       /* Update mirrored page tables for page table about to be freed */
> > +       int (*reflect_free_spt)(struct kvm *kvm, gfn_t gfn, enum pg_level
> > level,
> > +                               void *mirrored_spt);
> > +
> > +       /* Update mirrored page table from spte getting removed, and flush
> > TLB */
> > +       int (*reflect_remove_spte)(struct kvm *kvm, gfn_t gfn, enum pg_level
> > level,
> > +                                  kvm_pfn_t pfn);
> 
> Again, maybe free_external_spt and zap_external_spte?

Yep, I'm on board.

> 
> Also, please rename the last argument to pfn_for_gfn. I'm not proud of
> it, but it took me over 10 minutes to understand if the pfn referred
> to the gfn itself, or to the external SP that holds the spte...
> There's a possibility that it isn't just me. :)

Ah, I see how that could be confusing. 

> 
> (In general, this patch took me a _lot_ to review... there were a
> couple of places that left me incomprehensibly puzzled, more on this
> below).

Sorry for that. Thanks for taking the time to weed through it anyway.

> 
> >          bool (*has_wbinvd_exit)(void);
> > 
> >          u64 (*get_l2_tsc_offset)(struct kvm_vcpu *vcpu);
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 41b1d3f26597..1245f6a48dbe 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -346,6 +346,29 @@ static void tdp_mmu_unlink_sp(struct kvm *kvm, struct
> > kvm_mmu_page *sp)
> >          spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> >   }
> > 
> > +static void reflect_removed_spte(struct kvm *kvm, gfn_t gfn,
> > +                                       u64 old_spte, u64 new_spte,
> > +                                       int level)
> 
> new_spte is not used and can be dropped. Also, tdp_mmu_zap_external_spte?

Oh, yep. In v19 there used to be a KVM_BUG_ON(), but we can get rid of it.

> 
> > +{
> > +       bool was_present = is_shadow_present_pte(old_spte);
> > +       bool was_leaf = was_present && is_last_spte(old_spte, level);
> 
> Just put it below:
> 
> if (!is_shadow_present_pte(old_spte))
>   return;

Right, this is another miss from removing the KVM_BUG_ON()s.

> 
> /* Here we only care about zapping the external leaf PTEs. */
> if (!is_last_spte(old_spte, level))
> 
> > +       kvm_pfn_t old_pfn = spte_to_pfn(old_spte);
> > +       int ret;
> > +
> > +       /*
> > +        * Allow only leaf page to be zapped. Reclaim non-leaf page tables
> > page
> 
> This comment left me confused, so I'll try to rephrase and see if I
> can explain what happens. Correct me if I'm wrong.
> 
> The only paths to handle_removed_pt() are:
> - kvm_tdp_mmu_zap_leafs()
> - kvm_tdp_mmu_zap_invalidated_roots()
> 
> but because kvm_mmu_zap_all_fast() does not operate on mirror roots,
> the latter can only happen at VM destruction time.
> 
> But it's not clear why it's worth mentioning it here, or even why it
> is special at all. Isn't that just what handle_removed_pt() does at
> the end? Why does it matter that it's only done at VM destruction
> time?
> 
> In other words, it seems to me that this comment is TMI. And if I am
> wrong (which may well be), the extra information should explain the
> "why" in more detail, and it should be around the call to
> reflect_free_spt, not here.

TDX of course has the limitation around the ordering of the zapping S-EPT. So I
read the comment to be referring to how the implementation avoids zapping any
non-leaf PTEs during TD runtime.

But I'm going to have to circle back here after investigating a bit more. Isaku,
any comments on this comment and conditional?

> 
> > +               return;
> > +       /* Zapping leaf spte is allowed only when write lock is held. */
> > +       lockdep_assert_held_write(&kvm->mmu_lock);
> > +       /* Because write lock is held, operation should success. */
> > +       ret = static_call(kvm_x86_reflect_remove_spte)(kvm, gfn, level,
> > old_pfn);
> > +       KVM_BUG_ON(ret, kvm);
> > +}
> > +
> >   /**
> >    * handle_removed_pt() - handle a page table removed from the TDP
> > structure
> >    *
> > @@ -441,6 +464,22 @@ static void handle_removed_pt(struct kvm *kvm,
> > tdp_ptep_t pt, bool shared)
> >                  }
> >                  handle_changed_spte(kvm, kvm_mmu_page_as_id(sp), gfn,
> >                                      old_spte, REMOVED_SPTE, sp->role,
> > shared);
> > +               if (is_mirror_sp(sp)) {
> > +                       KVM_BUG_ON(shared, kvm);
> > +                       reflect_removed_spte(kvm, gfn, old_spte,
> > REMOVED_SPTE, level);
> > +               }
> > +       }
> > +
> > +       if (is_mirror_sp(sp) &&
> > +           WARN_ON(static_call(kvm_x86_reflect_free_spt)(kvm, sp->gfn, sp-
> > >role.level,
> > +                                                        
> > kvm_mmu_mirrored_spt(sp)))) {
> 
> Please use base_gfn and level here, instead of fishing them from sp.

Oh, yep, thanks.

> 
> > +               /*
> > +                * Failed to free page table page in mirror page table and
> > +                * there is nothing to do further.
> > +                * Intentionally leak the page to prevent the kernel from
> > +                * accessing the encrypted page.
> > +                */
> > +               sp->mirrored_spt = NULL;
> >          }
> > 
> >          call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
> > @@ -778,9 +817,11 @@ static u64 tdp_mmu_set_spte(struct kvm *kvm, int as_id,
> > tdp_ptep_t sptep,
> >          role.level = level;
> >          handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, role,
> > false);
> > 
> > -       /* Don't support setting for the non-atomic case */
> > -       if (is_mirror_sptep(sptep))
> > +       if (is_mirror_sptep(sptep)) {
> > +               /* Only support zapping for the non-atomic case */
> 
> Like for patch 10, this comment should point out why we never get here
> for mirror SPs.

Ok.