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

[Vishal Annapurve]

On Tue, Mar 18, 2025 at 8:42 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> 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@xxxxxxxxxx/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.
>