Re: [RFC PATCH V1 05/12] hvm/dm: Introduce xendevicemodel_set_irq_level DM op

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.08.2020 01:49, Stefano Stabellini wrote:
> On Thu, 6 Aug 2020, Julien Grall wrote:
>> On 06/08/2020 01:37, Stefano Stabellini wrote:
>>> On Wed, 5 Aug 2020, Julien Grall wrote:
>>>> On 05/08/2020 00:22, Stefano Stabellini wrote:
>>>>> On Mon, 3 Aug 2020, Oleksandr Tyshchenko wrote:
>>>>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>>>>>>
>>>>>> This patch adds ability to the device emulator to notify otherend
>>>>>> (some entity running in the guest) using a SPI and implements Arm
>>>>>> specific bits for it. Proposed interface allows emulator to set
>>>>>> the logical level of a one of a domain's IRQ lines.
>>>>>>
>>>>>> Please note, this is a split/cleanup of Julien's PoC:
>>>>>> "Add support for Guest IO forwarding to a device emulator"
>>>>>>
>>>>>> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
>>>>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>>>>>> ---
>>>>>>    tools/libs/devicemodel/core.c                   | 18
>>>>>> ++++++++++++++++++
>>>>>>    tools/libs/devicemodel/include/xendevicemodel.h |  4 ++++
>>>>>>    tools/libs/devicemodel/libxendevicemodel.map    |  1 +
>>>>>>    xen/arch/arm/dm.c                               | 22
>>>>>> +++++++++++++++++++++-
>>>>>>    xen/common/hvm/dm.c                             |  1 +
>>>>>>    xen/include/public/hvm/dm_op.h                  | 15
>>>>>> +++++++++++++++
>>>>>>    6 files changed, 60 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/tools/libs/devicemodel/core.c
>>>>>> b/tools/libs/devicemodel/core.c
>>>>>> index 4d40639..30bd79f 100644
>>>>>> --- a/tools/libs/devicemodel/core.c
>>>>>> +++ b/tools/libs/devicemodel/core.c
>>>>>> @@ -430,6 +430,24 @@ int xendevicemodel_set_isa_irq_level(
>>>>>>        return xendevicemodel_op(dmod, domid, 1, &op, sizeof(op));
>>>>>>    }
>>>>>>    +int xendevicemodel_set_irq_level(
>>>>>> +    xendevicemodel_handle *dmod, domid_t domid, uint32_t irq,
>>>>>> +    unsigned int level)
>>>>>
>>>>> It is a pity that having xen_dm_op_set_pci_intx_level and
>>>>> xen_dm_op_set_isa_irq_level already we need to add a third one, but from
>>>>> the names alone I don't think we can reuse either of them.
>>>>
>>>> The problem is not the name...
>>>>
>>>>>
>>>>> It is very similar to set_isa_irq_level. We could almost rename
>>>>> xendevicemodel_set_isa_irq_level to xendevicemodel_set_irq_level or,
>>>>> better, just add an alias to it so that xendevicemodel_set_irq_level is
>>>>> implemented by calling xendevicemodel_set_isa_irq_level. Honestly I am
>>>>> not sure if it is worth doing it though. Any other opinions?
>>>>
>>>> ... the problem is the interrupt field is only 8-bit. So we would only be
>>>> able
>>>> to cover IRQ 0 - 255.
>>>
>>> Argh, that's not going to work :-(  I wasn't sure if it was a good idea
>>> anyway.
>>>
>>>
>>>> It is not entirely clear how the existing subop could be extended without
>>>> breaking existing callers.
>>>>
>>>>> But I think we should plan for not needing two calls (one to set level
>>>>> to 1, and one to set it to 0):
>>>>> https://marc.info/?l=xen-devel&m=159535112027405
>>>>
>>>> I am not sure to understand your suggestion here? Are you suggesting to
>>>> remove
>>>> the 'level' parameter?
>>>
>>> My hope was to make it optional to call the hypercall with level = 0,
>>> not necessarily to remove 'level' from the struct.
>>
>> From my understanding, the hypercall is meant to represent the status of the
>> line between the device and the interrupt controller (either low or high).
>>
>> This is then up to the interrupt controller to decide when the interrupt is
>> going to be fired:
>>   - For edge interrupt, this will fire when the line move from low to high 
>> (or
>> vice versa).
>>   - For level interrupt, this will fire when line is high (assuming level
>> trigger high) and will keeping firing until the device decided to lower the
>> line.
>>
>> For a device, it is common to keep the line high until an OS wrote to a
>> specific register.
>>
>> Furthermore, technically, the guest OS is in charge to configure how an
>> interrupt is triggered. Admittely this information is part of the DT, but
>> nothing prevent a guest to change it.
>>
>> As side note, we have a workaround in Xen for some buggy DT (see the arch
>> timer) exposing the wrong trigger type.
>>
>> Because of that, I don't really see a way to make optional. Maybe you have
>> something different in mind?
> 
> For level, we need the level parameter. For edge, we are only interested
> in the "edge", right?

I don't think so, unless Arm has special restrictions. Edges can be
both rising and falling ones.

Jan