Re: [RFC PATCH 4/4] x86/vdso: x86/sgx: Allow the user to exit the vDSO loop on interrupts

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Aug 18, 2020 at 10:15:49AM -0700, Andy Lutomirski wrote:
> On Mon, Aug 17, 2020 at 9:24 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > Allow userspace to exit the vDSO on interrupts that are acknowledged
> > while the enclave is active.  This allows the user's runtime to switch
> > contexts at opportune times without additional overhead, e.g. when using
> > an M:N threading model (where M user threads run N TCSs, with N > M).
> 
> This is IMO rather odd.  We don't support this type of notification on
> interrupts for normal user code.  The fact user code can detect
> interrupts during enclave execution is IMO an oddity of SGX, and I
> have asked Intel to consider replacing the AEX mechanism with
> something more transparent to user mode.  If this ever happens, this
> mechanism is toast.
> 
> Even without architecture changes, building a *reliable* M:N threading
> mechanism on top of this will be difficult or impossible, as there is
> no particular guarantee that a thread will get timing interrupts at
> all or that these interrupts will get lucky and hit enclave code, thus
> triggering an AEX.  We certainly don't, and probably never will,
> support any corresponding feature for non-enclave code.
> 
> So this seems like an odd, and possibly unsupportable, feature to add.

I 100% agree that allowing the user to act on interrupts is weird/fragile.
I'll happily kill this off if there's an "official" NAK, but I wanted to
force the issue so that we're not stuck in limbo wondering whether or not
this should be supported.