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

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Message-ID: <b2d7227f960104f4bb53450c9820c1046589a049.camel@intel.com>
Date: Wed, 13 Aug 2025 00:51:06 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Annapurve, Vishal" <vannapurve@...gle.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"bp@...en8.de" <bp@...en8.de>, "kas@...nel.org" <kas@...nel.org>, "Huang,
 Kai" <kai.huang@...el.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "x86@...nel.org"
	<x86@...nel.org>, "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCHv2 00/12] TDX: Enable Dynamic PAMT

On Tue, 2025-08-12 at 17:18 -0700, Vishal Annapurve wrote:
> 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.

This is saying that the S-EPT can't be preserved, which doesn't really help me
understand why page allocations are such a big source of the work. I guess you
are saying it's the only thing possible to do?

Hmm, I'm not sure why the S-EPT couldn't be preserved, especially when you allow
for changes to KVM or the TDX module.

But if we are trying to solve the problem of making TD reboot faster, let's
figure out the biggest things that are making it slow first and work on that.
Like it's missing a lot of context on why this turned out to be the right
optimization to do.

Disclaimer: This optimization may be great for other types of VMs and that is
all well and good, but the points are about TDX here and the justification of
the TD reboot optimization is relevant to how we implement DPAMT.

> 
> > 
> > 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.

Still surprised that host page allocations turned out to be the biggest thing
sticking out.