Re: [PATCH 4/4] selftests/sgx: Trigger the reclaimer and #PF handler

[Reinette Chatre]

Hi Jarkko,

On 7/7/2021 1:50 PM, Jarkko Sakkinen wrote:
> On Wed, Jul 07, 2021 at 08:02:42AM -0700, Reinette Chatre wrote:
> > Hi Jarkko,
> >
> > On 7/7/2021 2:17 AM, Jarkko Sakkinen wrote:
> > > On Tue, Jul 06, 2021 at 05:10:38PM -0700, Reinette Chatre wrote:
> > > > Hi Jarkko,
> > > >
> > > > On 7/6/2021 4:50 PM, Jarkko Sakkinen wrote:
> > > > > On Tue, Jul 06, 2021 at 11:34:54AM -0700, Reinette Chatre wrote:
> > > > > > Hi Jarkko,
> > > > > >
> > > > > > On 7/5/2021 7:36 AM, Jarkko Sakkinen wrote:
> > > > > > > Create a heap for the test enclave, which has the same size as all
> > > > > > > available Enclave Page Cache (EPC) pages in the system. This will guarantee
> > > > > > > that all test_encl.elf pages *and* SGX Enclave Control Structure (SECS)
> > > > > > > have been swapped out by the page reclaimer during the load time. Actually,
> > > > > > > this adds a bit more stress than that since part of the EPC gets reserved
> > > > > > > for the Version Array (VA) pages.
> > > > > > >
> > > > > > > For each test, the page fault handler gets triggered in two occasions:
> > > > > > >
> > > > > > > - When SGX_IOC_ENCLAVE_INIT is performed, SECS gets swapped in by the
> > > > > > >       page fault handler.
> > > > > > > - During the execution, each page that is referenced gets swapped in
> > > > > > >       by the page fault handler.
> > > > > >
> > > > > > If I understand this correctly, all EPC pages are now being consumed during
> > > > > > fixture setup and thus every SGX test, no matter how big or small, now
> > > > > > becomes a stress test of the reclaimer instead of there being a unique
> > > > > > reclaimer test. Since an enclave is set up and torn down for every test this
> > > > > > seems like a significant addition. It also seems like this would impact
> > > > > > future tests of dynamic page addition where not all scenarios could be
> > > > > > tested with all EPC pages already consumed.
> > > > > >
> > > > > > Reinette
> > > > >
> > > > > Re-initializing the test enclave is mandatory thing to do for all tests
> > > > > because it has an internals state.
> > > >
> > > > Right, but not all tests require the same enclave. In kselftest terminology
> > > > I think you are attempting to force all tests to depend on the same test
> > > > fixture. Is it not possible to have a separate "reclaimer" test fixture that
> > > > would build an enclave with a large heap and then have reclaimer tests that
> > > > exercise it by being tests that are specific to this "reclaimer fixture"?
> > > >
> > > > Reinette
> > >
> > > Why add that complexity?
> >
> > With this change every test is turned into a pseudo reclaimer test without
> > there being any explicit testing (with pass/fail criteria) of reclaimer
> > behavior. This is an expensive addition and reduces the scenarios that the
> > tests can exercise.
> >
> > Reinette
>
> There is consistent known behaviour how reclaimer and also the page fault
> are exercised for each test. I think that is what matters most right now
> that the basic behaviour of both the page reclaimer and page fault handler
> gets exercised.

I believe the basic behavior of page fault handler is currently 
exercised in each test, this is required.

> I don't understand the real-world gain of doing something factors more
> complex than necessary at a particular point of time,  when you don't
> really need to hang yourself into it forever.

Your argument about "hang yourself into it forever" can go both ways - 
why should all tests now unnecessarily consume the entire EPC forever?

If I understand correctly adding a separate reclaimer test is not 
complex but would require refactoring code.

> This patch does increase the coverage in a deterministic manner to the code
> paths that were not previously exercised, i.e. we know the code paths, and
> could even calculate the exact number of times that they are triggered. And
> without doing anything obscure. That's what matters to me.

On the contrary this is indeed obfuscating the SGX tests: if an issue 
shows up in the reclaimer then all tests would fail. If there is a 
unique reclaimer test then that would help point to where the issue may be.

Reinette