Re: [PATCH v5 12/18] x86/sgx: Add EPC OOM path to forcefully reclaim EPC

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 16, 2023, Haitao Huang wrote:
> From this perspective, I think the current implementation is "well-defined":
> EPC cgroup limits for VMs are only enforced at VM launch time, not runtime.
> In practice,  SGX VM can be launched only with fixed EPC size and all those
> EPCs are fully committed to the VM once launched.

Fully committed doesn't mean those numbers are reflected in the cgroup.  A VM
scheduler can easily "commit" EPC to a guest, but allocate EPC on demand, i.e.
when the guest attempts to actually access a page.  Preallocating memory isn't
free, e.g. it can slow down guest boot, so it's entirely reasonable to have virtual
EPC be allocated on-demand.  Enforcing at launch time doesn't work for such setups,
because from the cgroup's perspective, the VM is using 0 pages of EPC at launch.

> Because of that, I imagine people are using VMs to primarily partition the
> physical EPCs, i.e, the static size itself is the 'limit' for the workload of
> a single VM and not expecting EPCs taken away at runtime.

If everything goes exactly as planned, sure.  But it's not hard to imagine some
configuration change way up the stack resulting in the hard limit for an EPC cgroup
being lowered.

> So killing does not really add much value for the existing usages IIUC.

As I said earlier, the behavior doesn't have to result in terminating a VM, e.g.
the virtual EPC code could provide a knob to send a signal/notification if the
owning cgroup has gone above the limit and the VM is targeted for forced reclaim.

> That said, I don't anticipate adding the enforcement of killing VMs at
> runtime would break such usages as admin/user can simply choose to set the
> limit equal to the static size to launch the VM and forget about it.
> 
> Given that, I'll propose an add-on patch to this series as RFC and have some
> feedback from community before we decide if that needs be included in first
> version or we can skip it until we have EPC reclaiming for VMs.

Gracefully *swapping* virtual EPC isn't required for oversubscribing virtual EPC.
Think of it like airlines overselling tickets.  The airline sells more tickets
than they have seats, and banks on some passengers canceling.  If too many people
show up, the airline doesn't swap passengers to the cargo bay, they just shunt them
to a different plane.

The same could be easily be done for hosts and virtual EPC.  E.g. if every VM
*might* use 1GiB, but in practice 99% of VMs only consume 128MiB, then it's not
too crazy to advertise 1GiB to each VM, but only actually carve out 256MiB per VM
in order to pack more VMs on a host.  If the host needs to free up EPC, then the
most problematic VMs can be migrated to a different host.

Genuinely curious, who is asking for EPC cgroup support that *isn't* running VMs?
AFAIK, these days, SGX is primarily targeted at cloud.  I assume virtual EPC is
the primary use case for an EPC cgroup.

I don't have any skin in the game beyond my name being attached to some of the
patches, i.e. I certainly won't stand in the way.  I just don't understand why
you would go through all the effort of adding an EPC cgroup and then not go the
extra few steps to enforce limits for virtual EPC.  Compared to the complexity
of the rest of the series, that little bit seems quite trivial.