On Tue, Jun 07, 2022 at 05:55:46PM -0700, Marc Orr wrote: > On Tue, Jun 7, 2022 at 12:01 AM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote: > > > > On Mon, Jun 06, 2022 at 01:09:50PM -0700, Vishal Annapurve wrote: > > > > > > > > Private memory map/unmap and conversion > > > > --------------------------------------- > > > > Userspace's map/unmap operations are done by fallocate() ioctl on the > > > > backing store fd. > > > > - map: default fallocate() with mode=0. > > > > - unmap: fallocate() with FALLOC_FL_PUNCH_HOLE. > > > > The map/unmap will trigger above memfile_notifier_ops to let KVM map/unmap > > > > secondary MMU page tables. > > > > > > > .... > > > > QEMU: https://github.com/chao-p/qemu/tree/privmem-v6 > > > > > > > > An example QEMU command line for TDX test: > > > > -object tdx-guest,id=tdx \ > > > > -object memory-backend-memfd-private,id=ram1,size=2G \ > > > > -machine q35,kvm-type=tdx,pic=no,kernel_irqchip=split,memory-encryption=tdx,memory-backend=ram1 > > > > > > > > > > There should be more discussion around double allocation scenarios > > > when using the private fd approach. A malicious guest or buggy > > > userspace VMM can cause physical memory getting allocated for both > > > shared (memory accessible from host) and private fds backing the guest > > > memory. > > > Userspace VMM will need to unback the shared guest memory while > > > handling the conversion from shared to private in order to prevent > > > double allocation even with malicious guests or bugs in userspace VMM. > > > > I don't know how malicious guest can cause that. The initial design of > > this serie is to put the private/shared memory into two different > > address spaces and gives usersapce VMM the flexibility to convert > > between the two. It can choose respect the guest conversion request or > > not. > > For example, the guest could maliciously give a device driver a > private page so that a host-side virtual device will blindly write the > private page. With this patch series, it's actually even not possible for userspace VMM to allocate private page by a direct write, it's basically unmapped from there. If it really wants to, it should so something special, by intention, that's basically the conversion, which we should allow. > > > It's possible for a usrspace VMM to cause double allocation if it fails > > to call the unback operation during the conversion, this may be a bug > > or not. Double allocation may not be a wrong thing, even in conception. > > At least TDX allows you to use half shared half private in guest, means > > both shared/private can be effective. Unbacking the memory is just the > > current QEMU implementation choice. > > Right. But the idea is that this patch series should accommodate all > of the CVM architectures. Or at least that's what I know was > envisioned last time we discussed this topic for SNP [*]. AFAICS, this series should work for both TDX and SNP, and other CVM architectures. I don't see where TDX can work but SNP cannot, or I missed something here? > > Regardless, it's important to ensure that the VM respects its memory > budget. For example, within Google, we run VMs inside of containers. > So if we double allocate we're going to OOM. This seems acceptable for > an early version of CVMs. But ultimately, I think we need a more > robust way to ensure that the VM operates within its memory container. > Otherwise, the OOM is going to be hard to diagnose and distinguish > from a real OOM. Thanks for bringing this up. But in my mind I still think userspace VMM can do and it's its responsibility to guarantee that, if that is hard required. By design, userspace VMM is the decision-maker for page conversion and has all the necessary information to know which page is shared/private. It also has the necessary knobs to allocate/free the physical pages for guest memory. Definitely, we should make userspace VMM more robust. Chao > > [*] https://lore.kernel.org/all/20210820155918.7518-1-brijesh.singh@xxxxxxx/ > > > > > Chao > > > > > > Options to unback shared guest memory seem to be: > > > 1) madvise(.., MADV_DONTNEED/MADV_REMOVE) - This option won't stop > > > kernel from backing the shared memory on subsequent write accesses > > > 2) fallocate(..., FALLOC_FL_PUNCH_HOLE...) - For file backed shared > > > guest memory, this option still is similar to madvice since this would > > > still allow shared memory to get backed on write accesses > > > 3) munmap - This would give away the contiguous virtual memory region > > > reservation with holes in the guest backing memory, which might make > > > guest memory management difficult. > > > 4) mprotect(... PROT_NONE) - This would keep the virtual memory > > > address range backing the guest memory preserved > > > > > > ram_block_discard_range_fd from reference implementation: > > > https://github.com/chao-p/qemu/tree/privmem-v6 seems to be relying on > > > fallocate/madvise. > > > > > > Any thoughts/suggestions around better ways to unback the shared > > > memory in order to avoid double allocation scenarios? > > I agree with Vishal. I think this patch set is making great progress. > But the double allocation scenario seems like a high-level design > issue that warrants more discussion.
