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Subject: Re: [RFC v2 00/27] Kernel Address Space Isolation
To:     Peter Zijlstra <peterz@infradead.org>
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From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Organization: Oracle Corporation
Message-ID: <8b84ac05-f639-b708-0f7f-810935b323e8@oracle.com>
Date:   Fri, 12 Jul 2019 15:46:19 +0200
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On 7/12/19 3:07 PM, Peter Zijlstra wrote:
> On Fri, Jul 12, 2019 at 02:47:23PM +0200, Alexandre Chartre wrote:
>> On 7/12/19 2:36 PM, Peter Zijlstra wrote:
>>> On Fri, Jul 12, 2019 at 02:17:20PM +0200, Alexandre Chartre wrote:
>>>> On 7/12/19 1:44 PM, Peter Zijlstra wrote:
>>>
>>>>> AFAIK3 this wants/needs to be combined with core-scheduling to be
>>>>> useful, but not a single mention of that is anywhere.
>>>>
>>>> No. This is actually an alternative to core-scheduling. Eventually, ASI
>>>> will kick all sibling hyperthreads when exiting isolation and it needs to
>>>> run with the full kernel page-table (note that's currently not in these
>>>> patches).
>>>>
>>>> So ASI can be seen as an optimization to disabling hyperthreading: instead
>>>> of just disabling hyperthreading you run with ASI, and when ASI can't preserve
>>>> isolation you will basically run with a single thread.
>>>
>>> You can't do that without much of the scheduler changes present in the
>>> core-scheduling patches.
>>>
>>
>> We hope we can do that without the whole core-scheduling mechanism. The idea
>> is to send an IPI to all sibling hyperthreads. This IPI will interrupt these
>> sibling hyperthreads and have them wait for a condition that will allow them
>> to resume execution (for example when re-entering isolation). We are
>> investigating this in parallel to ASI.
> 
> You cannot wait from IPI context, so you have to go somewhere else to
> wait.
> 
> Also, consider what happens when the task that entered isolation decides
> to schedule out / gets migrated.
> 
> I think you'll quickly find yourself back at core-scheduling.
> 

I haven't looked at details about what has been done so far. Hopefully, we
can do something not too complex, or reuse a (small) part of co-scheduling.

Thanks for pointing this out.

alex.