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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.07.24 23:53, Sean Christopherson wrote: > 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. I would mean consuming secretmem via the fd, *not* via page tables / gup. But if we also want to have the option of directmap modifications for shared memory in guest_memfd, we could make that indeed a guest_memfd feature. -- Cheers, David / dhildenb