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

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Message-ID: <aCclMAY0C5OXjt4/@yzhao56-desk.sh.intel.com>
Date: Fri, 16 May 2025 19:44:48 +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:
> > > +			goto retry;
> > > +
> > > +		sp = NULL;
> > > +		/*
> > > +		 * Set yielded in case after splitting to a lower level,
> > > +		 * the new iter requires furter splitting.
> > > +		 */
> > > +		iter.yielded = true;
> > > +		*flush = true;
> > > +	}
> > > +
> > > +	rcu_read_unlock();
> > > +
> > > +	/* Leave it here though it should be impossible for the mirror root */
> > > +	if (sp)
> > > +		tdp_mmu_free_sp(sp);
> > 
> > What do you think about relying on tdp_mmu_split_huge_pages_root() and moving
> > this to an optimization patch at the end?
> > 
> > Or what about just two calls to tdp_mmu_split_huge_pages_root() at the
> > boundaries?
> Though the two generally look like the same, relying on
> tdp_mmu_split_huge_pages_root() will create several minor changes scattering
> in tdp_mmu_split_huge_pages_root().
> 
> e.g. update flush after tdp_mmu_iter_cond_resched(), check
> iter_split_required(), set "iter.yielded = true".
> 
> So, it may be hard to review as a initial RFC.
> 
> I prefer to do that after Paolo and Sean have taken a look of it :)

Oh, I might misunderstood your meaning.
Yes, if necessary, we can provide a separate patch at the end to combine code of
tdp_mmu_split_huge_pages_root() and tdp_mmu_split_boundary_leafs().