Re: [RFC PATCH 0/4] KVM: ioctl for populating guest_memfd

David Hildenbrand
2024-11-20T15:13:13Z

>>
>> The questions are:
>> - Is this a well-known behaviour?
>> - Is there a way to mitigate that, ie make shared memory (including
>> guest_memfd) population faster/comparable to private memory?
>
> Likely. But your experiment measures above something different than what
> guest_memfd vs. anon does: guest_memfd doesn't update page tables, so I
> would assume guest_memfd will be faster than MAP_POPULATE.
>
> How do you end up allocating memory for guest_memfd? Using simple
> fallocate()?

Heh, now I spot that your comment was as reply to a series.

If your ioctl is supposed to to more than "allocating memory" like
MAP_POPULATE/MADV_POPULATE+* ... then POPULATE is a suboptimal choice.
Because for allocating memory, we would want to use fallocate() instead.
I assume you want to "allocate+copy"?

I'll note that, as we're moving into the direction of moving
guest_memfd.c into mm/guestmem.c, we'll likely want to avoid "KVM_*"
ioctls, and think about something generic.

Any clue how your new ioctl will interact with the WIP to have shared
memory as part of guest_memfd? For example, could it be reasonable to
"populate" the shared memory first (via VMA) and then convert that
"allocated+filled" memory to private?

--
Cheers,

David / dhildenb