linux-kernel - Re: [PATCH V4 1/1] KVM: TDX: Add sub-ioctl KVM_TDX_TERMINATE

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Message-ID: <CAGtprH8+iz1GqgPhH3g8jGA3yqjJXUF7qu6W6TOhv0stsa5Ohg@mail.gmail.com>
Date: Fri, 20 Jun 2025 14:21:52 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Hunter, Adrian" <adrian.hunter@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"Gao, Chao" <chao.gao@...el.com>, "Huang, Kai" <kai.huang@...el.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Chatre, Reinette" <reinette.chatre@...el.com>, 
	"Li, Xiaoyao" <xiaoyao.li@...el.com>, 
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>, 
	"tony.lindgren@...ux.intel.com" <tony.lindgren@...ux.intel.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: Re: [PATCH V4 1/1] KVM: TDX: Add sub-ioctl KVM_TDX_TERMINATE_VM

On Fri, Jun 20, 2025 at 11:59 AM Edgecombe, Rick P
<rick.p.edgecombe@...el.com> wrote:
>
> On Fri, 2025-06-20 at 07:24 -0700, Sean Christopherson wrote:
> > > The patch was tested with QEMU which AFAICT does not touch  memslots when
> > > shutting down.  Is there a reason to?
> >
> > In this case, the VMM process is not shutting down.  To emulate a reboot, the
> > VMM destroys the VM, but reuses the guest_memfd files for the "new" VM.
> > Because guest_memfd takes a reference to "struct kvm", through memslot
> > bindings, memslots need to be manually destroyed so that all references are
> > put and the VM is freed by the kernel.
>
> Sorry if I'm being dumb, but why does it do this? It saves freeing/allocating
> the guestmemfd pages? Or the in-place data gets reused somehow?

The goal is just to be able to reuse the same physical memory for the
next boot of the guest. Freeing and faulting-in the same amount of
memory is redundant and time-consuming for large VM sizes.

>
> The series Vishal linked has some kind of SEV state transfer thing. How is it
> intended to work for TDX?

The series[1] unblocks intrahost-migration [2] and reboot usecases.

[1] https://lore.kernel.org/lkml/cover.1747368092.git.afranji@google.com/#t
[2] https://lore.kernel.org/lkml/cover.1749672978.git.afranji@google.com/#t

>
> >   E.g. otherwise multiple reboots would manifest as memory leakds and
> > eventually OOM the host.
>
> This is in the case of future guestmemfd functionality? Or today?

Intrahost-migration and guest reboot are important usecases for Google
to support guest VM lifecycles.