Re: [Xen-devel] Ping: [PATCH] VMX: sync CPU state upon vCPU destruction

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 11/21/2017 04:42 PM, Dario Faggioli wrote:
> On Tue, 2017-11-21 at 08:29 -0700, Jan Beulich wrote:
>>>>> On 21.11.17 at 15:07, <igor.druzhinin@xxxxxxxxxx> wrote:
>>>
>> The question here is: In what other cases do we expect an RCU
>> callback to possibly touch guest state? I think the common use is
>> to merely free some memory in a delayed fashion.
>>
>>> Those choices that you outlined appear to be different in terms
>>> whether
>>> we solve the general problem and probably have some minor
>>> performance
>>> impact or we solve the ad-hoc problem but make the system more
>>> entangled. Here I'm more inclined to the first choice because this
>>> particular scenario the performance impact should be negligible.
>>
>> For the problem at hand there's no question about a
>> performance effect. The question is whether doing this for _other_
>> RCU callbacks would introduce a performance drop in certain cases.
>>
> Well, I personally favour the approach of making the piece of code that
> plays with the context responsible of not messing up when doing so.
> 
> And (replying to Igor comment above), I don't think that syncing
> context before RCU handlers solves the general problem --as you're
> calling it-- of "VMX code asynchronously messing up with the context". 
> In fzct, it solves the specific problem of "VMX code called via RCU,
> asynchronously messing up with the context".
> There may be other places where (VMX?) code messes with context, *not*
> from within an RCU handler, and that would still be an issue.

Yes, to expand on what I said earlier: Given that we cannot (at least
between now and the release) make it so that developers *never* have to
think about syncing state, it seems like the best thing to do is to make
coders *always* think about syncing state.  Syncing always in the RCU
handler means coders can get away sometimes without syncing; which makes
it more likely we'll forget in some other circumstance where it matters.

But that's my take on general principles; like Dario I wouldn't argue
too strongly if someone felt differently.

 -George

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