linux-kernel - Re: [PATCH RFC v1 3/5] KVM: guest_memfd: Call arch invalidation hooks when converting to shared

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Message-ID: <CAGtprH9Ucr-i_T7pdCFRq63GQY6nWy8668gFoG7NSGdLuzpzRg@mail.gmail.com>
Date: Wed, 16 Jul 2025 06:04:03 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Michael Roth <michael.roth@....com>
Cc: kvm@...r.kernel.org, linux-coco@...ts.linux.dev, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, david@...hat.com, tabba@...gle.com, 
	ackerleytng@...gle.com, ira.weiny@...el.com, thomas.lendacky@....com, 
	pbonzini@...hat.com, seanjc@...gle.com, vbabka@...e.cz, joro@...tes.org, 
	pratikrajesh.sampat@....com, liam.merwick@...cle.com, yan.y.zhao@...el.com, 
	aik@....com
Subject: Re: [PATCH RFC v1 3/5] KVM: guest_memfd: Call arch invalidation hooks
 when converting to shared

On Tue, Jul 15, 2025 at 3:56 PM Michael Roth <michael.roth@....com> wrote:
> > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > > index b77cdccd340e..f27e1f3962bb 100644
> > > --- a/virt/kvm/guest_memfd.c
> > > +++ b/virt/kvm/guest_memfd.c
> > > @@ -203,6 +203,28 @@ static int kvm_gmem_shareability_apply(struct inode *inode,
> > >         struct maple_tree *mt;
> > >
> > >         mt = &kvm_gmem_private(inode)->shareability;
> > > +
> > > +       /*
> > > +        * If a folio has been allocated then it was possibly in a private
> > > +        * state prior to conversion. Ensure arch invalidations are issued
> > > +        * to return the folio to a normal/shared state as defined by the
> > > +        * architecture before tracking it as shared in gmem.
> > > +        */
> > > +       if (m == SHAREABILITY_ALL) {
> > > +               pgoff_t idx;
> > > +
> > > +               for (idx = work->start; idx < work->start + work->nr_pages; idx++) {
> >
> > It is redundant to enter this loop for VM variants that don't need
> > this loop e.g. for pKVM/TDX. I think KVM can dictate a set of rules
> > (based on VM type) that guest_memfd will follow for memory management
> > when it's created, e.g. something like:
> > 1) needs pfn invalidation
> > 2) needs zeroing on shared faults
> > 3) needs zeroing on allocation
>
> Makes sense. Maybe internal/reserved GUEST_MEMFD_FLAG_*'s that can be passed
> to kvm_gmem_create()?

Yeah, a set of internal flags in addition to what is passed by user
space looks good to me. i.e. Something like:

-int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
+int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd
*args, u64 kvm_flags)

>
> -Mike