linux-kernel - Re: [PATCHv2 00/12] TDX: Enable Dynamic PAMT

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Message-ID: <CAGtprH8Mdgf_nx4qEN3eqp4KqmrW9OvxYHHDJDV2xa7nmBnGbA@mail.gmail.com>
Date: Tue, 12 Aug 2025 17:18:50 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Gao, Chao" <chao.gao@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"Huang, Kai" <kai.huang@...el.com>, "kas@...nel.org" <kas@...nel.org>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "bp@...en8.de" <bp@...en8.de>, "mingo@...hat.com" <mingo@...hat.com>, 
	"Zhao, Yan Y" <yan.y.zhao@...el.com>, "x86@...nel.org" <x86@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCHv2 00/12] TDX: Enable Dynamic PAMT

On Tue, Aug 12, 2025 at 4:35 PM Edgecombe, Rick P
<rick.p.edgecombe@...el.com> wrote:
>
> On Tue, 2025-08-12 at 15:00 -0700, Vishal Annapurve wrote:
> > IMO, tieing lifetime of guest_memfd folios with that of KVM ownership
> > beyond the memslot lifetime is leaking more state into guest_memfd
> > than needed. e.g. This will prevent usecases where guest_memfd needs
> > to be reused while handling reboot of a confidential VM [1].
>
> How does it prevent this? If you really want to re-use guest memory in a fast
> way then I think you would want the DPAMT to remain in place actually. It sounds
> like an argument to trigger the add/remove from guestmemfd actually.

With the reboot usecase, a new VM may start with it's own new HKID so
I don't think we can preserve any state that's specific to the
previous instance. We can only reduce the amount of state to be
maintained in SEPTs/DPAMTs by using hugepages wherever possible.

>
> But I really question with all the work to rebuild S-EPT, and if you propose
> DPAMT too, how much work is really gained by not needing to reallocate hugetlbfs
> pages. Do you see how it could be surprising? I'm currently assuming there is
> some missing context.

I would not limit the reboot usecase to just hugepage backing
scenario. guest_memfd folios (and ideally the guest_memfd files
themselves) simply should be reusable outside the VM lifecycle
irrespective of whether it's used to back CoCo VMs or not.