Re: [RFC PATCH 8/8] kvm: gmem: Allow restricted userspace mappings

[David Hildenbrand <david@redhat.com>]

>> Hi,
>>
>> sorry for the late reply. Yes, you could have joined .... too late.
> 
> No worries, I did end up joining to listen in to y'all's discussion
> anyway :)

Sorry for the late reply :(

> 
>> There will be a summary posted soon. So far the agreement is that we're
>> planning on allowing shared memory as part guest_memfd, and will allow
>> that to get mapped and pinned. Private memory is not going to get mapped
>> and pinned.
>>
>> If we have to disallow pinning of shared memory on top for some use
>> cases (i.e., no directmap), I assume that could be added.
>>
>>>
>>>> Note that just from staring at this commit, I don't understand the
>>>> motivation *why* we would want to do that.
>>>
>>> Fair - I admittedly didn't get into that as much as I probably should
>>> have. In our usecase, we do not have anything that pKVM would (I think)
>>> call "guest-private" memory. I think our memory can be better described
>>> as guest-owned, but always shared with the VMM (e.g. userspace), but
>>> ideally never shared with the host kernel. This model lets us do a lot
>>> of simplifying assumptions: Things like I/O can be handled in userspace
>>> without the guest explicitly sharing I/O buffers (which is not exactly
>>> what we would want long-term anyway, as sharing in the guest_memfd
>>> context means sharing with the host kernel), we can easily do VM
>>> snapshotting without needing things like TDX's TDH.EXPORT.MEM APIs, etc.
>>
>> Okay, so essentially you would want to use guest_memfd to only contain
>> shard memory and disallow any pinning like for secretmem.
> 
> Yeah, this is pretty much what I thought we wanted before listening in
> on Wednesday.
> 
> I've actually be thinking about this some more since then though. With
> hugepages, if the VM is backed by, say, 2M pages, our on-demand direct
> map insertion approach runs into the same problem that CoCo VMs have
> when they're backed by hugepages: How to deal with the guest only
> sharing a 4K range in a hugepage? If we want to restore the direct map
> for e.g. the page containing kvm-clock data, then we can't simply go
> ahead and restore the direct map for the entire 2M page, because there
> very well might be stuff in the other 511 small guest pages that we
> really do not want in the direct map. And we can't even take the

Right, you'd only want to restore the direct map for a fragment. Or 
dynamically map that fragment using kmap where required (as raised by 
Vlastimil).

> approach of letting the guest deal with the problem, because here
> "sharing" is driven by the host, not the guest, so the guest cannot
> possibly know that it maybe should avoid putting stuff it doesn't want
> shared into those remaining 511 pages! To me that sounds a lot like the
> whole "breaking down huge folios to allow GUP to only some parts of it"
> thing mentioned on Wednesday.

Yes. While it would be one logical huge page, it would be exposed to the 
remainder of the kernel as 512 individual pages.

> 
> Now, if we instead treat "guest memory without direct map entries" as
> "private", and "guest memory with direct map entries" as "shared", then
> the above will be solved by whatever mechanism allows gupping/mapping of
> only the "shared" parts of huge folios, IIUC. The fact that GUP is then
> also allowed for the "shared" parts is not actually a problem for us -
> we went down the route of disabling GUP altogether here because based on
> [1] it sounded like GUP for anything gmem related would never happen.

Right. Might there also be a case for removing the directmap for shared 
memory or is that not really a requirement so far?

> But after something is re-inserted into the direct map, we don't very
> much care if it can be GUP-ed or not. In fact, allowing GUP for the
> shared parts probably makes some things easier for us, as we can then do
> I/O without bounce buffers by just in-place converting I/O-buffers to
> shared, and then treating that shared slice of guest_memfd the same way
> we treat traditional guest memory today. 

Yes.

> In a very far-off future, we'd
> like to be able to do I/O without ever reinserting pages into the direct
> map, but I don't think adopting this private/shared model for gmem would
> block us from doing that?

How would that I/O get triggered? GUP would require the directmap.

> 
> Although all of this does hinge on us being able to do the in-place
> shared/private conversion without any guest involvement. Do you envision
> that to be possible?

Who would trigger the conversion and how? I don't see a reason why -- 
for your use case -- user space shouldn't be able to trigger conversion 
private <-> shared. At least nothing fundamental comes to mind that 
would prohibit that.

-- 
Cheers,

David / dhildenb