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

[Sean Christopherson <seanjc@google.com>]

On Tue, Oct 10, 2023, Haitao Huang wrote:
> On Mon, 09 Oct 2023 21:23:12 -0500, Huang, Kai <kai.huang@intel.com> wrote:
> 
> > On Mon, 2023-10-09 at 20:42 -0500, Haitao Huang wrote:
> > > Hi Sean
> > > 
> > > On Mon, 09 Oct 2023 19:23:04 -0500, Sean Christopherson
> > > <seanjc@google.com> wrote:
> > > > I can see userspace wanting to explicitly terminate the VM instead of
> > > > "silently"
> > > > the VM's enclaves, but that seems like it should be a knob in the
> > > > virtual EPC
> > > > code.
> > > 
> > > If my understanding above is correct and understanding your statement
> > > above correctly, then don't see we really need separate knob for vEPC
> > > code. Reaching a cgroup limit by a running guest (assuming dynamic
> > > allocation implemented) should not translate automatically killing
> > > the VM.
> > > Instead, it's user space job to work with guest to handle allocation
> > > failure. Guest could page and kill enclaves.
> > > 
> > 
> > IIUC Sean was talking about changing misc.max _after_ you launch SGX VMs:
> > 
> > 1) misc.max = 100M
> > 2) Launch VMs with total virtual EPC size = 100M	<- success
> > 3) misc.max = 50M
> > 
> > 3) will also succeed, but nothing will happen, the VMs will be still
> > holding 100M EPC.
> > 
> > You need to somehow track virtual EPC and kill VM instead.
> > 
> > (or somehow fail to do 3) if it is also an acceptable option.)
> > 
> Thanks for explaining it.
> 
> There is an error code to return from max_write. I can add that too to the
> callback definition and fail it when it can't be enforced for any reason.
> Would like some community feedback if this is acceptable though.

That likely isn't acceptable.  E.g. create a cgroup with both a host enclave and
virtual EPC, set the hard limit to 100MiB.  Virtual EPC consumes 50MiB, and the
host enclave consumes 50MiB.  Userspace lowers the limit to 49MiB.  The cgroup
code would reclaim all of the enclave's reclaimable EPC, and then kill the enclave
because it's still over the limit.  And then fail the max_write because the cgroup
is *still* over the limit.  So in addition to burning a lot of cycles, from
userspace's perspective its enclave was killed for no reason, as the new limit
wasn't actually set.

> I think to solve it ultimately, we need be able to adjust 'capacity' of VMs
> not to just kill them, which is basically the same as dynamic allocation
> support for VMs (being able to increase/decrease epc size when it is
> running). For now, we only have static allocation so max can't be enforced
> once it is launched.

No, reclaiming virtual EPC is not a requirement.  VMM EPC oversubscription is
insanely complex, and I highly doubt any users actually want to oversubcribe VMs.

There are use cases for cgroups beyond oversubscribing/swapping, e.g. privileged
userspace may set limits on a container to ensure the container doesn't *accidentally*
consume more EPC than it was allotted, e.g. due to a configuration bug that created
a VM with more EPC than it was supposed to have.  

My comments on virtual EPC vs. cgroups is much more about having sane, well-defined
behavior, not about saying the kernel actually needs to support oversubscribing EPC
for KVM guests.