Re: [PATCH] MMIO: Make coalesced mmio use a device per zone

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-07-20 10:24, Avi Kivity wrote:
> On 07/19/2011 08:23 PM, Jan Kiszka wrote:
>> On 2011-07-19 19:17, Avi Kivity wrote:
>>>  On 07/19/2011 08:14 PM, Jan Kiszka wrote:
>>>>
>>>>  Another improvement - unfortunately less transparent for user space -
>>>>  would be to overcome the single ring buffer that forces us to hold a
>>>>  central lock in user space while processing the entries. We rather need
>>>>  per-device rings. While waiting for coalesced VGA MMIO being processed,
>>>>  way too many kittens are killed.
>>>>
>>>>  I have this on our agenda, but I wouldn't be disappointed as well if
>>>>  someone else is faster.
>>>
>>>  The socket mmio would have accomplished this as well.
>>
>> I haven't followed the outcome in all details - is that approach dead
>> due to its complexity?
>>
>>>  One thing to
>>>  beware of is to preserve correctness:
>>>
>>>  1) write to 0xa0000 (queued)
>>>  2) write to 0xa0002 (queued)
>>>  3) remap 0xa0000 region (executed)
>>
>> Obviously, there must be 3a) here: drain all affected queues.
> 
> How do you implement this 3a, if your consumers are outside the main 
> process?  I guess you could have an additional synchonize API (for 
> in-kernel consumers) or RPC (for external process consumers), but then 
> this is no longer a simple API.

I'm not planning to leave the hypervisor process for now, not to speak
of in-kernel models. Already for many other reasons, a synchronization
API between a hypothetical decoupled device model and the core will be
quite complex. The first step is to get it scalable using a single process.

Jan

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