linux-kernel - Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for TDP MMU

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Date: Thu, 23 May 2024 23:14:07 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>
CC: "sagis@...gle.com" <sagis@...gle.com>, "dmatlack@...gle.com"
	<dmatlack@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>, "Aktas,
 Erdem" <erdemaktas@...gle.com>
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
 TDP MMU

On Tue, 2024-05-14 at 17:59 -0700, Rick Edgecombe wrote:
> +static void handle_removed_private_spte(struct kvm *kvm, gfn_t gfn,
> +                                       u64 old_spte, u64 new_spte,
> +                                       int level)
> +{
> +       bool was_present = is_shadow_present_pte(old_spte);
> +       bool was_leaf = was_present && 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
> +        * at destroying VM.
> +        */
> +       if (!was_leaf)
> +               return;
> +
> +       /* Zapping leaf spte is allowed only when write lock is held. */
> +       lockdep_assert_held_write(&kvm->mmu_lock);
> +       ret = static_call(kvm_x86_zap_private_spte)(kvm, gfn, level);
> +       /* Because write lock is held, operation should success. */
> +       if (KVM_BUG_ON(ret, kvm))
> +               return;
> +
> +       ret = static_call(kvm_x86_remove_private_spte)(kvm, gfn, level,
> old_pfn);

I don't see why these (zap_private_spte and remove_private_spte) can't be a
single op. Was it to prepare for huge pages support or something? In the base
series they are both only called once.

> +       KVM_BUG_ON(ret, kvm);
> +}
> +