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

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Date: Tue, 28 May 2024 21:48:45 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Yamahata, Isaku" <isaku.yamahata@...el.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "sagis@...gle.com"
	<sagis@...gle.com>, "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
 TDP MMU

On Fri, 2024-05-24 at 01:20 -0700, Isaku Yamahata wrote:
> > 
> > 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.
> 
> That is for large page support. The step to merge or split large page is
> 1. zap_private_spte()
> 2. tlb shoot down
> 3. merge/split_private_spte()

I think we can simplify it for now. Otherwise we can't justify it without
getting into the huge page support.

Looking at how to create some more explainable code here, I'm also wondering
about the tdx_track() call in tdx_sept_remove_private_spte(). I didn't realize
it will send IPIs to each vcpu for *each* page getting zapped. Another one in
the "to optimize later" bucket I guess. And I guess it won't happen very often.