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

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Message-ID: <39cc767dd2f3792f1b36e224f1567dfb997fc0cf.camel@intel.com>
Date: Fri, 16 May 2025 22:27:10 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "Shutemov, Kirill" <kirill.shutemov@...el.com>,
	"Hansen, Dave" <dave.hansen@...el.com>, "david@...hat.com"
	<david@...hat.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "tabba@...gle.com" <tabba@...gle.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "michael.roth@....com"
	<michael.roth@....com>, "seanjc@...gle.com" <seanjc@...gle.com>, "Weiny, Ira"
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	<chao.p.peng@...el.com>, "Du, Fan" <fan.du@...el.com>,
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	<jun.miao@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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Subject: Re: [RFC PATCH 16/21] KVM: x86/mmu: Introduce
 kvm_split_boundary_leafs() to split boundary leafs

On Fri, 2025-05-16 at 16:03 +0800, Yan Zhao wrote:
> > 
> > > > +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().

Yea, lack of consistency would raise other questions.

> 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));
>  

Hmm, let's be clear about the logic. This is essentially a mirror TDP only
function, and there we don't have the same invalid root scenarios as the more
complicated cases. I'm not exactly sure how we could hit the warning if they
didn't match. I guess a hole punch on the fd while the TD is getting torn down?

Let's comment the reasoning at least.