linux-kernel - Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

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Message-ID: <aEtumIYPJSV49_jL@google.com>
Date: Thu, 12 Jun 2025 17:19:36 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Kai Huang <kai.huang@...el.com>, 
	"quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Fan Du <fan.du@...el.com>, Dave Hansen <dave.hansen@...el.com>, 
	"david@...hat.com" <david@...hat.com>, "thomas.lendacky@....com" <thomas.lendacky@....com>, 
	"vbabka@...e.cz" <vbabka@...e.cz>, Zhiquan1 Li <zhiquan1.li@...el.com>, 
	Kirill Shutemov <kirill.shutemov@...el.com>, Yan Y Zhao <yan.y.zhao@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>, 
	"michael.roth@....com" <michael.roth@....com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, 
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "tabba@...gle.com" <tabba@...gle.com>, 
	Chao P Peng <chao.p.peng@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, Vishal Annapurve <vannapurve@...gle.com>, 
	"jroedel@...e.de" <jroedel@...e.de>, Jun Miao <jun.miao@...el.com>, 
	"pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

On Thu, Jun 12, 2025, Rick P Edgecombe wrote:
> On Wed, 2025-06-11 at 07:42 -0700, Sean Christopherson wrote:
> > If there's a *legitimate* use case where the guest wants to ACCEPT a subset of
> > memory, then there should be an explicit TDCALL to request that the unwanted
> > regions of memory be unmapped.  Smushing everything into implicit behavior has
> > obvioulsy created a giant mess.
> 
> Hi, still digging on if there is any possible use.
> 
> I think this may need a guest opt-in, so the guest can say it can handle errors
> for both smaller and larger page size matches. So it may not matter if there is
> a rare usage or not. If KVM finds the guest opts-in (how to do that TBD), it can
> start mapping at the host level. 

Hmm, clever.  That should work; requiring an updated guest kernel to get optimal
performance doesn't seem too onerous.

> If KVM doesn't see the opt-in, the guest gets 4k pages.

As in, KVM doesn't even try to use hugepage mappings?  If so, this idea probably
gets my vote.