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 18:16:09 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"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 Tue, May 28, 2024 at 09:48:45PM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> 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.

Ok. Now we don't care large page support, we can combine those hooks into single
hook.


> 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.

We need it. Without tracking (or TLB shoot down), we'll hit
TDX_TLB_TRACKING_NOT_DONE.  The TDX module has to guarantee that there is no
remaining TLB entries for pages freed by TDH.MEM.PAGE.REMOVE().
-- 
Isaku Yamahata <isaku.yamahata@...el.com>