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

[David Hildenbrand (Arm)]

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?

If it is a longterm pin, it must use FOLL_LONGTERM. :/

I assume we fail in check_vma_flags()

	if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))
		return -EOPNOTSUPP;

IIRC, fsdax cannot tolerate unbounded pins. Is that the case we are running into?

How does vfio deal with that? (does it?)

-- 
Cheers,

David