linux-kernel - Re: [PATCH v2 10/15] KVM: x86/tdp_mmu: Reflect building mirror page tables

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Date: Fri, 7 Jun 2024 20:52:29 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dmatlack@...gle.com" <dmatlack@...gle.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
Subject: Re: [PATCH v2 10/15] KVM: x86/tdp_mmu: Reflect building mirror page
 tables

On Fri, 2024-06-07 at 12:10 +0200, Paolo Bonzini wrote:
> On Thu, May 30, 2024 at 11:07 PM Rick Edgecombe
> <rick.p.edgecombe@...el.com> wrote:
> > +       /* Update mirrored mapping with page table link */
> > +       int (*reflect_link_spt)(struct kvm *kvm, gfn_t gfn, enum pg_level
> > level,
> > +                               void *mirrored_spt);
> > +       /* Update the mirrored page table from spte getting set */
> > +       int (*reflect_set_spte)(struct kvm *kvm, gfn_t gfn, enum pg_level
> > level,
> > +                               kvm_pfn_t pfn);
> 
> Possibly link_external_spt and set_external_spte, since you'll have to
> s/mirrored/external/ in the comment. But not a hard request.

Definitely seems better now that we have the "external" nomenclature.

> 
> > +static void *get_mirrored_spt(gfn_t gfn, u64 new_spte, int level)
> > +{
> > +       if (is_shadow_present_pte(new_spte) && !is_last_spte(new_spte,
> > level)) {
> > +               struct kvm_mmu_page *sp =
> > to_shadow_page(pfn_to_hpa(spte_to_pfn(new_spte)));
> 
> I think this is spte_to_child_sp(new_spte)?

Yes, that seems much easier than all this.


[snip]

> 
> > +static int __must_check reflect_set_spte_present(struct kvm *kvm,
> > tdp_ptep_t sptep,
> 
> tdp_mmu_set_mirror_spte_atomic?
> 
> > +       /*
> > +        * For mirrored page table, callbacks are needed to propagate SPTE
> > +        * change into the mirrored page table. In order to atomically
> > update
> > +        * both the SPTE and the mirrored page tables with callbacks,
> > utilize
> > +        * freezing SPTE.
> > +        * - Freeze the SPTE. Set entry to REMOVED_SPTE.
> > +        * - Trigger callbacks for mirrored page tables.
> > +        * - Unfreeze the SPTE.  Set the entry to new_spte.
> > +        */
> 
> /*
>  * We need to lock out other updates to the SPTE until the external
>  * page table has been modified. Use REMOVED_SPTE similar to
>  * the zapping case.
>  */
> 
> Easy peasy. :) We may want to rename REMOVED_SPTE to FROZEN_SPTE; feel
> free to do it at the head of this series, then it can be picked for
> 6.11.

Ok.

> 
> > -static inline int __tdp_mmu_set_spte_atomic(struct tdp_iter *iter, u64
> > new_spte)
> > +static inline int __tdp_mmu_set_spte_atomic(struct kvm *kvm, struct
> > tdp_iter *iter, u64 new_spte)
> >  {
> >         u64 *sptep = rcu_dereference(iter->sptep);
> > 
> > @@ -571,15 +629,36 @@ static inline int __tdp_mmu_set_spte_atomic(struct
> > tdp_iter *iter, u64 new_spte)
> >          */
> >         WARN_ON_ONCE(iter->yielded || is_removed_spte(iter->old_spte));
> > 
> > -       /*
> > -        * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
> > -        * does not hold the mmu_lock.  On failure, i.e. if a different
> > logical
> > -        * CPU modified the SPTE, try_cmpxchg64() updates iter->old_spte
> > with
> > -        * the current value, so the caller operates on fresh data, e.g. if
> > it
> > -        * retries tdp_mmu_set_spte_atomic()
> > -        */
> > -       if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte))
> > -               return -EBUSY;
> > +       if (is_mirror_sptep(iter->sptep) && !is_removed_spte(new_spte)) {
> > +               int ret;
> > +
> > +               /* Don't support atomic zapping for mirrored roots */
> 
> The why is hidden in the commit message to patch 11. I wonder if it
> isn't clearer to simply squash together patches 10 and 11 (your call),
> and instead split out the addition of the new struct kvm parameters.

I actually split them in two for this v2. I thought the combined patch was too
big. Maybe I could just move this whole hunk to the next patch. I'll give it a
try.

> 
> Anyway, this comment needs a bit more info:
> 
> /*
>  * Users of atomic zapping don't operate on mirror roots,
>  * so only need to handle present new_spte.
>  */

Ok.

> 
> > +               if (KVM_BUG_ON(!is_shadow_present_pte(new_spte), kvm))
> > +                       return -EBUSY;
> > +               /*
> > +                * Populating case.
> > +                * - reflect_set_spte_present() implements
> > +                *   1) Freeze SPTE
> > +                *   2) call hooks to update mirrored page table,
> > +                *   3) update SPTE to new_spte
> > +                * - handle_changed_spte() only updates stats.
> > +                */
> 
> Comment not needed (weird I know).

Sure.