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

[Elliot Berman]

On 8/9/2022 4:29 AM, Marc Zyngier wrote:
> On Mon, 08 Aug 2022 23:22:48 +0100,
> Elliot Berman <quic_eberman at 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.
  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.


> Thanks,
> 
> 	M.
>