Re: [PATCH v7 11/14] KVM: Register/unregister the guest private memory regions

Sean Christopherson
2022-07-29T19:54:16Z

On Mon, Jul 25, 2022, Chao Peng wrote:
> On Thu, Jul 21, 2022 at 05:58:50PM +0000, Sean Christopherson wrote:
> > On Thu, Jul 21, 2022, Chao Peng wrote:
> > > On Thu, Jul 21, 2022 at 03:34:59PM +0800, Wei Wang wrote:
> > > >
> > > >
> > > > On 7/21/22 00:21, Sean Christopherson wrote:
> > > > Maybe you could tag it with cgs for all the confidential guest support
> > > > related stuff: e.g. kvm_vm_ioctl_set_cgs_mem()
> > > >
> > > > bool is_private = ioctl == KVM_MEMORY_ENCRYPT_REG_REGION;
> > > > ...
> > > > kvm_vm_ioctl_set_cgs_mem(, is_private)
> > >
> > > If we plan to widely use such abbr. through KVM (e.g. it's well known),
> > > I'm fine.
> >
> > I'd prefer to stay away from "confidential guest", and away from any VM-scoped
> > name for that matter. User-unmappable memmory has use cases beyond hiding guest
> > state from the host, e.g. userspace could use inaccessible/unmappable memory to
> > harden itself against unintentional access to guest memory.
> >
> > > I actually use mem_attr in patch: https://lkml.org/lkml/2022/7/20/610
> > > But I also don't quite like it, it's so generic and sounds say nothing.
> > >
> > > But I do want a name can cover future usages other than just
> > > private/shared (pKVM for example may have a third state).
> >
> > I don't think there can be a third top-level state. Memory is either private to
> > the guest or it's not. There can be sub-states, e.g. memory could be selectively
> > shared or encrypted with a different key, in which case we'd need metadata to
> > track that state.
> >
> > Though that begs the question of whether or not private_fd is the correct
> > terminology. E.g. if guest memory is backed by a memfd that can't be mapped by
> > userspace (currently F_SEAL_INACCESSIBLE), but something else in the kernel plugs
> > that memory into a device or another VM, then arguably that memory is shared,
> > especially the multi-VM scenario.
> >
> > For TDX and SNP "private vs. shared" is likely the correct terminology given the
> > current specs, but for generic KVM it's probably better to align with whatever
> > terminology is used for memfd. "inaccessible_fd" and "user_inaccessible_fd" are
> > a bit odd since the fd itself is accesible.
> >
> > What about "user_unmappable"? E.g.
> >
> > F_SEAL_USER_UNMAPPABLE, MFD_USER_UNMAPPABLE, KVM_HAS_USER_UNMAPPABLE_MEMORY,
> > MEMFILE_F_USER_INACCESSIBLE, user_unmappable_fd, etc...
>
> For KVM I also think user_unmappable looks better than 'private', e.g.
> user_unmappable_fd/KVM_HAS_USER_UNMAPPABLE_MEMORY sounds more
> appropriate names. For memfd however, I don't feel that strong to change
> it from current 'inaccessible' to 'user_unmappable', one of the reason
> is it's not just about unmappable, but actually also inaccessible
> through direct ioctls like read()/write().

Heh, I _knew_ there had to be a catch. I agree that INACCESSIBLE is better for
memfd.