linux-kernel - Re: [RFC PATCH 16/21] KVM: x86/mmu: Introduce kvm_split_boundary

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Message-ID: <aCbxSyLUhjyeB+05@yzhao56-desk.sh.intel.com>
Date: Fri, 16 May 2025 16:03:23 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>, "Shutemov,
 Kirill" <kirill.shutemov@...el.com>, "quic_eberman@...cinc.com"
	<quic_eberman@...cinc.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Hansen, Dave"
	<dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "tabba@...gle.com"
	<tabba@...gle.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>, "Du, Fan"
	<fan.du@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "michael.roth@....com"
	<michael.roth@....com>, "Weiny, Ira" <ira.weiny@...el.com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "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>, "jroedel@...e.de"
	<jroedel@...e.de>, "Miao, Jun" <jun.miao@...el.com>, "pgonda@...gle.com"
	<pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 16/21] KVM: x86/mmu: Introduce
 kvm_split_boundary_leafs() to split boundary leafs

On Fri, May 16, 2025 at 03:46:53PM +0800, Yan Zhao wrote:
> On Wed, May 14, 2025 at 06:56:26AM +0800, Edgecombe, Rick P wrote:
> > On Thu, 2025-04-24 at 11:08 +0800, Yan Zhao wrote:

> > >  /*
> > >   * If can_yield is true, will release the MMU lock and reschedule if the
> > >   * scheduler needs the CPU or there is contention on the MMU lock. If this
> > > @@ -991,6 +1006,8 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> > >  		    !is_last_spte(iter.old_spte, iter.level))
> > >  			continue;
> > >  
> > > +		WARN_ON_ONCE(iter_split_required(kvm, root, &iter, start, end));
> > > +
> > 
> > Kind of unrelated change? But good idea. Maybe for another patch.
> Yes, will move it to a separate patch in a formal version.
> As initial RFC, I hoped to show related changes in one patch to allow a whole
> picture.
> 
> > > +int kvm_tdp_mmu_gfn_range_split_boundary(struct kvm *kvm, struct kvm_gfn_range *range)
> > > +{
> > > +	enum kvm_tdp_mmu_root_types types;
> > > +	struct kvm_mmu_page *root;
> > > +	bool flush = false;
> > > +	int ret;
> > > +
> > > +	types = kvm_gfn_range_filter_to_root_types(kvm, range->attr_filter) | KVM_INVALID_ROOTS;
> > 
> > What is the reason for KVM_INVALID_ROOTS in this case?
> I wanted to keep consistent with that in kvm_tdp_mmu_unmap_gfn_range().
With this consistency, we can warn in tdp_mmu_zap_leafs() as below though
there should be no invalid mirror root.

WARN_ON_ONCE(iter_split_required(kvm, root, &iter, start, end));