Re: [Xen-devel] Xen/arm: Virtual ITS command queue handling

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 5 May 2015, Vijay Kilari wrote:
> 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

Good. Is it possible to actually assign an LPI to a domain when/if a PCI
device is assigned to the domain? So that we don't waste LPIs for
domains that are not going to use the vITS?


> - 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
>        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
> 4) Append INT command with completion_irq of current domain

I would consider adding a vcpu_block call


> 5) Release vITS lock
> 6) Take physical ITS (pITS) lock
> 7) Write translated cmds to physical ITS
> 8) Add entry in its_requests
> 9) Release pITS lock
> 10) return from trap
>
> One receiving completion interrupt:
> 
> 1) Take the first pending request from its_requests.
> 2) Update vITS CREADER of the guest indicating completion of command to guest

I would add vcpu_unblock


> 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.
>    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.
> 
> 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

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