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

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Date:   Wed, 7 Jul 2021 14:20:04 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
        linux-sgx@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] selftests/sgx: Trigger the reclaimer and #PF handler

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