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.
> Heh, this was going to be my exact question about why it was safe to drop 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.
> No need for a v3, I'll add a blurb when applying.

Thanks, Sean!

Best regards,

Pankaj