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

["Gupta, Pankaj" <pankaj.gupta@amd.com>]

Hi David,

>> 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;

Yes, it fails in this path but for file backed mapping, vma_is_fsdax() 
returns false because

vma_is_dax() returns false:

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

Host side backend is regular file backed memory (no fsdax).

Thanks,

Pankaj

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