Re: [RFC PATCH v2 13/22] xen/arm: its: Add virtual ITS command support

[Julien Grall <julien.grall@citrix.com>]

Hi Vijay,

On 04/05/2015 16:19, Vijay Kilari wrote:
>>>>> How did you implement the interrupt mode? Could it be improve?
>>>>
>>>>
>>>>      1) In physical ITS driver its_device is created with devID 00:00.1
>>>> with
>>>> 256 MSI-x are reserved and is named as completion_dev, which is global.
>>>
>>>
>>> That's a lot of MSI-x reserved... Can't you use only one per domain?
>>
>>
>> Hmmm... I meant for all the domain, not "per domain".
>
>     Complexity with one irq for all domains is that if completion interrupt
> comes it is difficult to find out  for which vITS/Domain ITS command
> it came for.

While reserving a single devID sounds feasible on all the future 
platform. Allocating 256 MSI-x sounds more difficult, you assume that 
any board will have at least 256 MSI-x free.

Although, this is not scalable. How do you plan to handle more than 256 
domains? By increasing the number of reserved MSI-x?

I don't ask you to implement the later now... but if increasing the 
number of domain supported means rewriting all the completion code and 
maybe the vITS then you should ask yourself if it's really worth to take 
this current approach.

[..]

>>>>    I am adding one INT per command. This can be improved to add one INT
>>>> cmd for all
>>>>    the pending commands. Existing Linux driver sends 2 commands at a time.
>>>
>>>
>>> You should not assume that other OS will send 2 commands at the same
>>> time... It could be more or less.
>>>
>>> Although, having a INT per command is rather slow. One INT command per
>>> batch would improve the boot time.
>
>     We cannot limit on number of commands sent at a time. we have to send all the
> pending commands in vITS queue at a time when trapped on CWRITER, Otherwise
> we have to check for pending interrupts on completion interrupt and translate
> and send pending commands in interrupt context. Which complicates and adds more
> delays.

If we don't limit the number of commands sent, we would allow a domain 
to flood the command queue. Therefore, other domains wouldn't be able to 
send command and will likely timeout and crash. This is one possible 
security issue among many others.

Nobody like security issue, it impacts both end-user and the project. 
Please have this security concern in mind before performance. 
Performance is usually more easier to address later.

As the vITS is only used for interrupt managing (mapping, unmapping), 
it's not used in hot path such as receiving interrupt. So we don't care 
if it's "slow" from the guest point of view as long as we emulate the 
behavior correctly without impacting the other domain.

Also, what happen if the physical queue is full? You need to have a away 
to inject new commands later.

Overall I'm aware that the command queue emulation is huge. Lots of 
things to take into account : security, performance, concurrence problem...

As you did before in this thread, I would suggest you to write down all 
the possible solutions and see what are the impacts (security, theorical 
performances...). So we could talk on the ML (or over an IRC/phone 
meeting) and agree to a solution that satisfy everyone.

By experience, doing a such things may speed up the acceptance of the 
series because everyone will focus on the implementation when you send a 
new patch.

One good example is x86 PML support. Intel has sent a design doc a 
couple of months ago [1]. Developers discussed about the overall design, 
when a common agreement has been made they send the patch series.

The same would have been very helpful to understand this series. TBH, I 
spent most of my time trying to understand what was your design and how 
everything works together. On a 4000 lines series split in 22 patches 
it's a rather big task.

Although I don't necessarily ask for exactly the same. It could be part 
of the cover letter and/or commit messages.

Regards,

[1] http://www.gossamer-threads.com/lists/xen/devel/366537

-- 
Julien Grall