On 7/9/24 22:13, David Hildenbrand wrote: > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe. > > > > On 09.07.24 16:48, Fuad Tabba wrote: >> Hi Patrick, >> >> On Tue, Jul 9, 2024 at 2:21 PM Patrick Roy <roypat@xxxxxxxxxxxx> wrote: >>> >>> Allow mapping guest_memfd into userspace. Since AS_INACCESSIBLE is set >>> on the underlying address_space struct, no GUP of guest_memfd will be >>> possible. >> >> This patch allows mapping guest_memfd() unconditionally. Even if it's >> not guppable, there are other reasons why you wouldn't want to allow >> this. Maybe a config flag to gate it? e.g., > > > As discussed with Jason, maybe not the direction we want to take with > guest_memfd. > If it's private memory, it shall not be mapped. Also not via magic > config options. > > We'll likely discuss some of that in the meeting MM tomorrow I guess > (having both shared and private memory in guest_memfd). Oh, nice. I'm assuming you mean this meeting: https://lore.kernel.org/linux-mm/197a2f19-c71c-fbde-a62a-213dede1f4fd@xxxxxxxxxx/T/? Would it be okay if I also attend? I see it also mentions huge pages, which is another thing we are interested in, actually :) > 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. > -- > Cheers, > > David / dhildenb >
