Re: [PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration

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

>> 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. It also kept
>> FOLL_WRITE on the pin.
>>
>> Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed
>> guest memory, such as virtio-pmem host memory-backend-file mappings
>> (MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file
>> mappings 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").
>>
>> Region registration only requires long-term pin to prevent page migration and
>> does not write through this GUP pin.
>>
>> Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM.
> Worth mentioning here something like
>
> "In the past, FOLL_WRITE was required to trigger CoW unsharing, making sure that
> we don't end up replacing the page in the page tables during a later write fault
> after already having pinned a (shared) page in MAP_PRIVATE mappings.
> FOLL_LONGTERM does that nowadays (see gup_must_unshare()) even without FOLL_WRITE.
>
> Given that SEV only pins RAM for XYZ and doesn't actually write to the pinned
> pages, we can just drop the FOLL_WRITE"
>
> Fill out XYZ :)

Sure :)

I will update the commit message in v3.

>
> In general, LGTM
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

thanks

Pankaj