On Wed, Jul 10, 2024, David Hildenbrand wrote: > On 10.07.24 11:51, Patrick Roy wrote: > > On 7/9/24 22:13, David Hildenbrand wrote: > > > 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. > > If so, I wonder if it wouldn't be better to simply add KVM support to > consume *real* secretmem memory? IIRC so far there was only demand to > probably remove the directmap of private memory in guest_memfd, not of > shared memory. It's also desirable to remove shared memory from the directmap, e.g. to prevent using the directmap in a cross-VM attack. I don't think we want to allow KVM to consume secretmem. That would require letting KVM gup() secretmem, which AIUI defeats the entire purpose of secretmem, and I don't think KVM should be special.
