On Wed, Dec 19, 2018 at 07:00:47AM +0200, Jarkko Sakkinen wrote:
> On Tue, Dec 18, 2018 at 10:53:49AM -0800, Sean Christopherson wrote:
> > What if we re-organize the ioctls in such a way that we leave open the
> > possibility of allocating raw EPC for KVM via /dev/sgx? I'm not 100%
> > positive this approach will work[1], but conceptually it fits well with
> > KVM's memory model, e.g. KVM is aware of the GPA<->HVA association but
> > generally speaking doesn't know what's physically backing each memory
> > region.
>
> Why would you want to pass EPC through user space to KVM rather than
> KVM allocating it through kernel interfaces?
Delegating EPC management to userspace fits better with KVM's existing
memory ABI. KVM provides a single ioctl(), KVM_SET_USER_MEMORY_REGION[1],
that allows userspace to create, move, modify and delete memory regions.
Skipping over a lot of details, there are essentially three options for
exposing EPC to a KVM guest:
1) Provide a dedicated KVM ioctl() to manage EPC without routing it
through KVM_SET_USER_MEMORY_REGION.
2) Add a flag to 'struct kvm_userspace_memory_region' that denotes an
EPC memory region and mmap() / allocate EPC in KVM.
3) Provide an ABI to allocate raw EPC and let userspace manage it like
any other memory region.
Option (1) requires duplicating all of KVM_SET_USER_MEMORY_REGION's
functionality unless the ioctl() is severly restricted.
Option (2) is an ugly abuse of KVM_SET_USER_MEMORY_REGION since the EPC
flag would have completely different semantics than all other usage of
KVM_SET_USER_MEMORY_REGION.
Thus, option (3).
Probably a better question to answer is why provide the ABI through
/dev/sgx and not /dev/kvm. IMO /dev/sgx is a more logical way to
advertise support to userspace, e.g. userspace can simply check if
/dev/sgx (or /dev/sgx/epc) exists vs. probing a KVM capability.
Without EPC oversubscription in KVM, /dev/sgx is easily the best fit
since the EPC management would reside completely in x86/sgx, i.e. KVM
would essentially have zero code related to EPC management.
EPC oversubscription complicates things because the architecture forces
aspects of VMM oversubscription into the KVM domain, e.g. requires a
post-VMXON instruction (ENCLV) and a VM-Exit handler. I still think
/dev/sgx is a better fit, my only concern is that the oversubscription
code would be even more heinous due to splitting responsibilities.
But, Andy's idea of having /dev/sgx/enclave vs. /dev/sgx/epc might help
avoid that entirely.
