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

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Message-ID: <aEyj_5WoC-01SPsV@google.com>
Date: Fri, 13 Jun 2025 15:19:43 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Yan Y Zhao <yan.y.zhao@...el.com>, Fan Du <fan.du@...el.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Kai Huang <kai.huang@...el.com>, 
	"quic_eberman@...cinc.com" <quic_eberman@...cinc.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>, "michael.roth@....com" <michael.roth@....com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Chao P Peng <chao.p.peng@...el.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, Ira Weiny <ira.weiny@...el.com>, 
	Isaku Yamahata <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>, 
	Vishal Annapurve <vannapurve@...gle.com>, "tabba@...gle.com" <tabba@...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 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.

All the more reason to make this a property of the VM that is passed via
"struct td_params".  I.e. put the onus on the owner of the VM to ensure their
kernel(s) have been updated accordingly.

I understand that this could be painful, but honestly _all_ of TDX and SNP is
painful for the guest.  E.g. I don't think it's any worse than the security
issues with TDX (and SNP) guests using kvmclock (which I'd love some reviews on,
btw).

https://lore.kernel.org/all/20250227021855.3257188-35-seanjc@google.com