Re: [PATCH v10 12/26] gunyah: vm_mgr: Add/remove user memory regions

[Sean Christopherson <seanjc@google.com>]

On Fri, Feb 24, 2023, Elliot Berman wrote:
> 
> 
> On 2/24/2023 2:19 AM, Fuad Tabba wrote:
> > Hi,
> > 
> > On Tue, Feb 14, 2023 at 9:26 PM Elliot Berman <quic_eberman@quicinc.com> wrote:
> > > 
> > > 
> > > When launching a virtual machine, Gunyah userspace allocates memory for
> > > the guest and informs Gunyah about these memory regions through
> > > SET_USER_MEMORY_REGION ioctl.
> > 
> > I'm working on pKVM [1], and regarding the problem of donating private
> > memory to a guest, we and others working on confidential computing
> > have faced a similar issue that this patch is trying to address. In
> > pKVM, we've initially taken an approach similar to the one here by
> > pinning the pages being donated to prevent swapping or migration [2].
> > However, we've encountered issues with this approach since the memory
> > is still mapped by the host, which could cause the system to crash on
> > an errant access.
> > 
> > Instead, we've been working on adopting an fd-based restricted memory
> > approach that was initially proposed for TDX [3] and is now being
> > considered by others in the confidential computing space as well
> > (e.g., Arm CCA [4]). The basic idea is that the host manages the guest
> > memory via a file descriptor instead of a userspace address. It cannot
> > map that memory (unless explicitly shared by the guest [5]),
> > eliminating the possibility of the host trying to access private
> > memory accidentally or being tricked by a malicious actor. This is
> > based on memfd with some restrictions. It handles swapping and
> > migration by disallowing them (for now [6]), and adds a new type of
> > memory region to KVM to accommodate having an fd representing guest
> > memory.
> > 
> > Although the fd-based restricted memory isn't upstream yet, we've
> > ported the latest patches to arm64 and made changes and additions to
> > make it work with pKVM, to test it and see if the solution is feasible
> > for us (it is). I wanted to mention this work in case you find it
> > useful, and in the hopes that we can all work on confidential
> > computing using the same interfaces as much as possible.
> 
> Thanks for highlighting the memfd_restricted changes to us! We'll
> investigate how/if it can suit Gunyah usecases.

Can you provide Gunyah's requirements/rules and use cases as they relate to memory
management?  I agree with Fuad, this is pretty much exactly what memfd_restricted()
is intended to handle.  If Gunyah has a unique requirement or use case, it'd be
helpful to find out sooner than later.  E.g.

  1. What is the state of memory when it's accepted by a VM?  Is it undefined,
     i.e. the VM's responsibility to initialize?  If not, is it always
     zero-initialized or can memory be populated by the RM?

  2. When exclusive/private memory is reclaimed, can the VM's data be preserved,
     or is it unconditionally

  3. How frequently is memory transition allocated/reclaimed?

  4. Are there assumptions and/or limitations on the size or granlarity of
     memory objects?

  5. Can memory be shared by multiple VMs but _not_ be accessible from the RM?

  6. etc. :-)

Thanks!