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

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Message-ID: <cbee132077fd59f181d1fc19670b72a51f2d9fa1.camel@intel.com>
Date: Fri, 13 Jun 2025 21:53:08 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "Du, Fan" <fan.du@...el.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"Huang, Kai" <kai.huang@...el.com>, "quic_eberman@...cinc.com"
	<quic_eberman@...cinc.com>, "Hansen, Dave" <dave.hansen@...el.com>,
	"david@...hat.com" <david@...hat.com>, "thomas.lendacky@....com"
	<thomas.lendacky@....com>, "vbabka@...e.cz" <vbabka@...e.cz>, "Li, Zhiquan1"
	<zhiquan1.li@...el.com>, "Shutemov, Kirill" <kirill.shutemov@...el.com>,
	"michael.roth@....com" <michael.roth@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"Peng, Chao P" <chao.p.peng@...el.com>, "pbonzini@...hat.com"
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	<isaku.yamahata@...el.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Annapurve, Vishal" <vannapurve@...gle.com>, "tabba@...gle.com"
	<tabba@...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 09/21] KVM: TDX: Enable 2MB mapping size after TD is
 RUNNABLE

On Fri, 2025-06-13 at 09:32 +0800, Yan Zhao wrote:
> > > 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.
> > 
> > Fair.
> 
> Agreed.

Arg, I just realized a one-way opt-in will have a theoretical gap. If the guest
kexec's, the new kernel will need to match the opt-in.

A full solution could allow a later opt-out that is handled by the VMM by
shattering all page tables. But it starts to get too complex I think. Especially
since Linux guests already try to accept in the order 1GB->2MB->4k. So in
practice we are already worrying about correctness and not functional issues.
Maybe we just ignore it.

Otherwise, we currently have the following requirements I think:
1. One-way guest opt-in to new TDG.MEM.PAGE.ACCEPT behavior
2. Some notification to KVM that the guest has opted in.
3. After opt-in, TDG.MEM.PAGE.ACCEPT will return TDX_PAGE_SIZE_MISMATCH if
mapping is too small or too big

Thinking about how we would like the notification... Maybe we could have the
actual behavior controlled by the host, and have some GHCI like communication
like a TDVMCALL. The TDVMCALL (or similar) could be handled within KVM.
Basically just call the host side opt-in.

The reason to have it host controllable is that, as above, the new behavior
should be fine for a normal Linux guest. A host user controlled opt-in could be
useful for anyone that wants to run huge pages for old guest kernels. A kvm
module param maybe.

If this sounds good, I'll get the TDX modules input and come back with some
specific spec.