Re: [PATCH 5/7] x86/sgx: Add flag to zero added region instead of copying from source

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

On Mon, Jun 10, 2019 at 11:09:50AM -0700, Xing, Cedric wrote:
> > From: Andy Lutomirski [mailto:luto@amacapital.net]
> > Sent: Friday, June 07, 2019 12:32 PM
> > 
> > > On Jun 6, 2019, at 10:32 AM, Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> > >> On Thu, Jun 06, 2019 at 10:20:38AM -0700, Andy Lutomirski wrote:
> > >> On Wed, Jun 5, 2019 at 12:49 PM Sean Christopherson
> > >> <sean.j.christopherson@intel.com> wrote:
> > >>>
> > >>> For some enclaves, e.g. an enclave with a small code footprint and a
> > >>> large working set, the vast majority of pages added to the enclave
> > >>> are zero pages.  Introduce a flag to denote such zero pages.  The
> > >>> major benefit of the flag will be realized in a future patch to use
> > >>> Linux's actual zero page as the source, as opposed to explicitly
> > >>> zeroing the enclave's backing memory.
> > >>>
> > >>
> > >> I feel like I probably asked this at some point, but why is there a
> > >> workqueue here at all?
> > >
> > > Performance.  A while back I wrote a patch set to remove the worker
> > > queue and discovered that it tanks enclave build time when the enclave
> > > is being hosted by a Golang application.  Here's a snippet from a mail
> > > discussing the code.
> > >
> > >    The bad news is that I don't think we can remove the add page
> > worker
> > >    as applications with userspace schedulers, e.g. Go's M:N scheduler,
> > >    can see a 10x or more throughput improvement when using the worker
> > >    queue.  I did a bit of digging for the Golang case to make sure I
> > >    wasn't doing something horribly stupid/naive and found that it's a
> > >    generic issue in Golang with blocking (or just long-running) system
> > >    calls.  Because Golang multiplexes Goroutines on top of OS threads,
> > >    blocking syscalls introduce latency and context switching overhead,
> > >    e.g. Go's scheduler will spin up a new OS thread to service other
> > >    Goroutines after it realizes the syscall has blocked, and will
> > later
> > >    destroy one of the OS threads so that it doesn't build up too many
> > >    unused.
> > >
> > > IIRC, the scenario is spinning up several goroutines, each building an
> > > enclave.  I played around with adding a flag to do a synchronous EADD
> > > but didn't see a meaningful change in performance for the simple case.
> > > Supporting both the worker queue and direct paths was complex enough
> > > that I decided it wasn't worth the trouble for initial upstreaming.
> > 
> > Sigh.
> > 
> > It seems silly to add a workaround for a language that has trouble
> > calling somewhat-but-not-too-slow syscalls or ioctls.
> > 
> > How about fixing this in Go directly?  Either convince the golang people
> > to add a way to allocate a real thread for a particular region of code
> > or have the Go SGX folks write a bit of C code to do  a whole bunch of
> > ioctls and have Go call *that*.  Then the mess stays in Go where it
> > belongs.
> 
> The add page worker is less about performance but more about concurrency
> restrictions in SGX ISA. EADD/EEXTEND are slow instructions and both take
> lock on the SECS. So if there's another EADD/EEXTEND in progress then it will
> #GP.
> 
> In practice, no sane applications will EADD in multiple threads because that
> would make MRENCLAVE unpredictable. But adversary may use that just to
> trigger #GP in kernel. Therefore, the SGX module would have to lock around
> EADD/EEXTEND anyway. And then we figured it would be better to have the add
> page worker so that no lock would be necessary, plus (small) improvement in
> performance. 

That may have been true years ago, but it doesn't reflect the current
reality.  The ADD_PAGE ioctl *and* the worker function both take the
enclave's lock.  The worker function actually takes it twice, once to pull
the request off the queue, and once to do EADD/EEXTEND.  The lock is
temporarily released by the worker function to avoid deadlock in case EPC
page allocation triggers reclaim.