Re: [PATCH] KVM: SEV: drop FOLL_LONGTERM for encrypted region registration

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 01, 2026, David Hildenbrand (Arm) wrote:
> On 7/1/26 18:30, Sean Christopherson wrote:
> > On Wed, Jul 01, 2026, David Hildenbrand (Arm) wrote:
> >> On 7/1/26 16:45, Pankaj Gupta wrote:
> >>> commit 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering encrypted memory regions")
> >>> added FOLL_LONGTERM to sev_mem_enc_register_region() so anonymous guest RAM is
> >>> migrated out of MIGRATE_CMA/ZONE_MOVABLE before a long term pin. This breaks
> >>> virtio-pmem which has file backed (MAP_SHARED) host mapping where GUP rejects
> >>> FOLL_WRITE | FOLL_LONGTERM since:
> >>>
> >>> commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings")
> >>> commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings").
> >>>
> >>> Drop FOLL_LONGTERM when registering encrypted memory regions and restore
> >>> the previous behavior.
> >>
> >> But that breaks the original issue of breaking ZONE_MOVABLE/CMA?
> > 
> > Ya.
> > 
> >> If it is a longterm pin, it must use FOLL_LONGTERM. :/
> > 
> > Heh, well, KVM showed that that's not entirely true for many years :-)
> 
> What exactly do you mean? KVM MMUs sync through memory notifiers and doesn't
> need this.
> 
> It's only our "interesting" CoCo code :)

Yeah, I'm just being cheeky and saying that it's obviously possible to do what
is effectively a long-term pint without specifying FOLL_LONGTERM, i.e. saying
it "must" use FOLL_LONGTERM isn't super duper strictly true.