Re: Xen/arm: Virtual ITS command queue handling

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

On 05/05/15 13:14, Vijay Kilari wrote:
> Hi,
> 

Hi Vijay,

>    As discussed, here is the design doc/txt.

I will comment on the proposal 2 as it seems to be the preferred one
assuming you are able to find why it's slow.

> Proposal 2:
> ----------------
> Here when guest writes command to vITS queue and updates CWRITER registers,
> it is trapped in XEN and below steps are followed to process ITS command
> 
> - Dom0 creates a ITS completion device with device id (00:00.1) and reserves
>   n number (256 or so) irqs (LPIs) for this device.
> - One irq/LPI (called completion_irq) of this completion device is
> allocated per domain
> - With this irq/LPI descriptor we can identify the domain/vITS.
> - Info of all the ongoing ITS requests(put in pITS Queue) of this domain is
>   stored in ITS command status array (called its_requests). This is
> managed per vITS.
> 
> 1) Trap of CWRITER write by guest
> 2) Take vITS lock
> 3) Read all the commands written by guest, translate it
>     - If one of the guest command is INT command

Why do you need a specific handling for the guest INT command?

>        a) Append INT command with completion_irq and write this batch as
>           seperate request and goto (3) to process next commands
>     - If more than 'n' commands are sent by guest, start a timer to process
>       remaining commands

Hmmm... How are you sure the time for the timer would be enough?

> 4) Append INT command with completion_irq of current domain
> 5) Release vITS lock
> 6) Take physical ITS (pITS) lock
> 7) Write translated cmds to physical ITS
> 8) Add entry in its_requests

You don't explain what is its_requests.

> 9) Release pITS lock
> 10) return from trap
> 
> One receiving completion interrupt:
> 
> 1) Take the first pending request from its_requests.

I'm assuming that you have some kind of array/list to store the pending
request? I think this would be more difficult to manage than only
supporting one batch per domain at any time.

> 2) Update vITS CREADER of the guest indicating completion of command to guest
> 
> Cons:
>    - Has overhead of processing completion interrupt.
>    - Need to reserve a fake device to generate completion interrupt and
>      reserve one LPI per-domain
> 
> Pros:
>    - VCPU does not poll in Xen for completion of commands.
>    - Handles guest flooding command queue with commands. But needs timer
> 
> Handling Command queue state:
>  - Physical Queue cannot be full as it 64KB there by it can accomodate
> 1K ITS commands.

I don't understand this sentence. Why do you think the physical queue
cannot be full?

>    In case it is full, VCPU has to poll with timeout till physical
> Queue is empty before it post
>    next command
>  - If vITS Queue condition should be managed by guest ITS driver.

Same here.

> Behaviour of Polling and completion interrupt based guest driver:
>  - If completion interrupt (INT) is used by guest driver, then insert
> Xen completion
>    INT command so that CREADER is updated before guest's INT command is injected
>  - If polling mode is used, trap on CREADER checks for completion of command
> 

Regards,

-- 
Julien Grall