Re: [PATCH v2 07/11] gunyah: msgq: Add Gunyah message queues

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

On 09/08/2022 19:50, Elliot Berman wrote:
> 
> 
> On 8/9/2022 4:29 AM, Marc Zyngier wrote:
>> On Mon, 08 Aug 2022 23:22:48 +0100,
>> Elliot Berman <quic_eberman@quicinc.com> wrote:
>>>
>>> In a future series, I'll add the support to load other virtual
>>> machines. When running other virtual machines, additional gunyah
>>> devices are needed for doorbells (e.g. to emulate interrupts for
>>> paravirtualized devices) and to represent the vCPUs of that other
>>> VM. Other gunyah devices are also possible, but those are the
>>> immediate devices coming over the horizon.
>>
>> Can you elaborate on this "doorbell" aspect? If you signal interrupts
>> to guests, they should be signalled as actual interrupts, not as some
>> hypervisor-specific events, as we rely on the interrupt semantics for
>> most things.
>>
>> Or are you talking about injecting an interrupt from a guest into
>> another, the doorbell representing an interrupt source?
>>
> 
> Doorbells can operate either of these modes:
>   1. As simple interrupt sources. The doorbell sender makes a hypercall
>      and an interrupt is raised on the receiver. The hypervisor can be
>      configured to raise a specific SPI on the receiver VM and simply
>      acknowledging the SPI is enough to clear the interrupt assert. No
>      hypervisor-specific code is needed on the receiver to handle these
>      interrupts. This is the mode one would expect to use for
>      paravirtualized devices.

This sounds good.

>   2. As hypervisor-specific events which must be acknowledged using
>      hypercalls. We aren't currently using this advanced use-case and no
>      plans currently to post these. However, I can try to briefly
>      explain: These doorbells can operate on a bitfield and the sender
>      can assert flags on the bitmask; the receiver can decide which bits
>      should trigger the interrupt and which SPI the doorbell "runs" on.
>      The "user story" for this doorbell is to support multiple sender
>      using the same doorbell object. Each sender has a few designated
>      bits they should set. The receiver can choose which events it wants
>      an interrupt to be raised for and then can process all the pending
>      events. To re-iterate, we don't have an interesting use-case for
>      this yet, so don't plan on post patches for this second mode of
>      doorbell.

Well. For me this sounds like 'we have such capability, no real usecase, 
but we want to support it anyway' kind of story. As history has shown 
multiple times, the order should be the opposite one. First you have the 
use case, then you create the API for it. Otherwise it is very easy to 
end up with the abstraction that looks good on the API side, but is very 
hard to fit into the actual user code.

I would suggest to drop the second bullet for now and focus on getting 
the simple doorbells done and accepted into mainline.

-- 
With best wishes
Dmitry