linux-kernel - Re: [PATCH RFC] KVM: TDX: Defer guest memory removal to decrease shutdown time

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Message-ID: <CAGtprH_9nQ02Phd0RLcq7YDVOE7VKuVkvNnFktfpAsVuBUc+Ow@mail.gmail.com>
Date: Tue, 18 Mar 2025 14:11:01 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, pbonzini@...hat.com, seanjc@...gle.com, 
	kvm@...r.kernel.org, rick.p.edgecombe@...el.com, kai.huang@...el.com, 
	reinette.chatre@...el.com, xiaoyao.li@...el.com, 
	tony.lindgren@...ux.intel.com, binbin.wu@...ux.intel.com, 
	isaku.yamahata@...el.com, linux-kernel@...r.kernel.org, yan.y.zhao@...el.com, 
	chao.gao@...el.com
Subject: Re: [PATCH RFC] KVM: TDX: Defer guest memory removal to decrease
 shutdown time

On Tue, Mar 18, 2025 at 8:42 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
>
> On 17/03/25 10:13, Kirill A. Shutemov wrote:
> > On Thu, Mar 13, 2025 at 08:16:29PM +0200, Adrian Hunter wrote:
> >> @@ -3221,6 +3241,19 @@ int tdx_gmem_private_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn)
> >>      return PG_LEVEL_4K;
> >>  }
> >>
> >> +int tdx_gmem_defer_removal(struct kvm *kvm, struct inode *inode)
> >> +{
> >> +    struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> >> +
> >> +    if (kvm_tdx->nr_gmem_inodes >= TDX_MAX_GMEM_INODES)
> >> +            return 0;
> >
> > We have graceful way to handle this, but should we pr_warn_once() or
> > something if we ever hit this limit?
> >
> > Hm. It is also a bit odd that we need to wait until removal to add a link
> > to guest_memfd inode from struct kvm/kvm_tdx. Can we do it right away in
> > __kvm_gmem_create()?
>
> Sure.
>
> The thing is, the inode is currently private within virt/kvm/guest_memfd.c
> so there needs to be a way to make it accessible to arch code.  Either a
> callback passes it, or it is put on struct kvm in some way.
>
> >
> > Do I read correctly that inode->i_mapping->i_private_list only ever has
> > single entry of the gmem? Seems wasteful.
>
> Yes, it is presently used for only 1 gmem.

Intrahost migration support[1] will make use of this list to track
additional linked files. We are planning to float a next revision
soon.

[1] https://lore.kernel.org/lkml/ZN%2F81KNAWofRCaQK@google.com/t/

>
> >
> > Maybe move it to i_private (I don't see flags being used anywhere) and
> > re-use the list_head to link all inodes of the struct kvm?
> >
> > No need in the gmem_inodes array.
>
> There is also inode->i_mapping->i_private_data which is unused.
>