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

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Message-ID: <aEt0ZxzvXngfplmN@google.com>
Date: Thu, 12 Jun 2025 17:44:23 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Kai Huang <kai.huang@...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>, Yan Y Zhao <yan.y.zhao@...el.com>, 
	Zhiquan1 Li <zhiquan1.li@...el.com>, Kirill Shutemov <kirill.shutemov@...el.com>, 
	"michael.roth@....com" <michael.roth@....com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, Chao P Peng <chao.p.peng@...el.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, 
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "vbabka@...e.cz" <vbabka@...e.cz>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
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	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 Fri, Jun 13, 2025, Rick P Edgecombe wrote:
> On Thu, 2025-06-12 at 17:19 -0700, Sean Christopherson wrote:
> > > 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.
> 
> Maybe an "I can handle it" accept size bit that comes in the exit qualification?

Eww, no.  Having to react on _every_ EPT violation would be annoying, and trying
to debug issues where the guest is mixing options would probably be a nightmare.

I was thinking of something along the lines of an init-time or boot-time opt-in.

> Yan, do you see any problems with that? Like if a guest passed it in some accept
> and not others? Thinking about the new "unaccept" SEAMCALL...