linux-kernel - Re: [RFC PATCH v2 09/23] KVM: x86/tdp_mmu: Add split_external_spt hook called during write mmu

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Message-ID: <aRVNggcp4Gr078ha@yzhao56-desk.sh.intel.com>
Date: Thu, 13 Nov 2025 11:16:18 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Li,
 Xiaoyao" <xiaoyao.li@...el.com>, "Du, Fan" <fan.du@...el.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "tabba@...gle.com" <tabba@...gle.com>, "kas@...nel.org"
	<kas@...nel.org>, "michael.roth@....com" <michael.roth@....com>, "Weiny, Ira"
	<ira.weiny@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"Peng, Chao P" <chao.p.peng@...el.com>, "Annapurve, Vishal"
	<vannapurve@...gle.com>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"Miao, Jun" <jun.miao@...el.com>, "x86@...nel.org" <x86@...nel.org>,
	"pgonda@...gle.com" <pgonda@...gle.com>
Subject: Re: [RFC PATCH v2 09/23] KVM: x86/tdp_mmu: Add split_external_spt
 hook called during write mmu_lock

On Tue, Nov 11, 2025 at 06:06:47PM +0800, Huang, Kai wrote:
> On Thu, 2025-08-07 at 17:43 +0800, Yan Zhao wrote:
> > Introduce the split_external_spt hook and call it within tdp_mmu_set_spte()
> > for the mirror page table.
> 
> Nit: I think you need to use split_external_spt() since it's a function,
> even you already mentioned it is a hook.
Makes sense.

> > tdp_mmu_set_spte() is invoked for SPTE transitions under write mmu_lock.
> > For the mirror page table, in addition to the valid transitions from a
> > shadow-present entry to !shadow-present entry, introduce a new valid
> > transition case for splitting and propagate the transition to the external
> > page table via the hook split_external_spt.
> 
> Ditto: split_external_spt()
Will update. 

> [...]
> 
> >  static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(bool mirror);
> > +static void *get_external_spt(gfn_t gfn, u64 new_spte, int level);
> 
> Is it possible to get rid of such declarations, e.g., by ...
I think so.

Will drop this declaration by moving split_external_spt() after
get_external_spt() but before set_external_spte_present() and
tdp_mmu_set_spte().

Thanks for this suggestion.

> >  static void tdp_account_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
> >  {
> > @@ -384,6 +385,18 @@ static void remove_external_spte(struct kvm *kvm, gfn_t gfn, u64 old_spte,
> >  	KVM_BUG_ON(ret, kvm);
> >  }
> >  
> > +static int split_external_spt(struct kvm *kvm, gfn_t gfn, u64 old_spte,
> > +			      u64 new_spte, int level)
> > +{
> > +	void *external_spt = get_external_spt(gfn, new_spte, level);
> > +	int ret;
> > +
> > +	KVM_BUG_ON(!external_spt, kvm);
> > +
> > +	ret = kvm_x86_call(split_external_spt)(kvm, gfn, level, external_spt);
> > +
> > +	return ret;
> > +}
> 
> ... moving split_external_spt() somewhere else, e.g., after
> set_external_spte_present() (which calls get_external_spt())?
> 
> Since ...
> 
> >  /**
> >   * handle_removed_pt() - handle a page table removed from the TDP structure
> >   *
> > @@ -765,12 +778,20 @@ static u64 tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
> >  	handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level, false);
> >  
> >  	/*
> > -	 * Users that do non-atomic setting of PTEs don't operate on mirror
> > -	 * roots, so don't handle it and bug the VM if it's seen.
> > +	 * Propagate changes of SPTE to the external page table under write
> > +	 * mmu_lock.
> > +	 * Current valid transitions:
> > +	 * - present leaf to !present.
> > +	 * - present non-leaf to !present.
> > +	 * - present leaf to present non-leaf (splitting)
> >  	 */
> >  	if (is_mirror_sptep(sptep)) {
> > -		KVM_BUG_ON(is_shadow_present_pte(new_spte), kvm);
> > -		remove_external_spte(kvm, gfn, old_spte, level);
> > +		if (!is_shadow_present_pte(new_spte))
> > +			remove_external_spte(kvm, gfn, old_spte, level);
> > +		else if (is_last_spte(old_spte, level) && !is_last_spte(new_spte, level))
> > +			split_external_spt(kvm, gfn, old_spte, new_spte, level);
> > +		else
> > +			KVM_BUG_ON(1, kvm);
> >  	}
> > 
> 
> ... split_external_spt() is only called here in tdp_mmu_set_spte() which is
> way after set_external_spte_present() AFAICT.