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

[Jan Beulich <jbeulich@suse.com>]

On 18.08.2020 00:56, Stefano Stabellini wrote:
> On Mon, 17 Aug 2020, Jan Beulich wrote:
>> On 07.08.2020 23:50, Stefano Stabellini wrote:
>>> On Fri, 7 Aug 2020, Jan Beulich wrote:
>>>> 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@epam.com>
>>>>>>>>>>
>>>>>>>>>> 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@arm.com>
>>>>>>>>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>>>>>>>> ---
>>>>>>>>>>    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.
>>>
>>> And the same is true for level interrupts too: they could be active-low
>>> or active-high.
>>>
>>>
>>> Instead of modelling the state of the line, which seems to be a bit
>>> error prone especially in the case of a single-device emulator that
>>> might not have enough information about the rest of the system (it might
>>> not know if the interrupt is active-high or active-low), we could model
>>> the triggering of the interrupt instead.
>>>
>>> In the case of level=1, it would mean that the interrupt line is active,
>>> no matter if it is active-low or active-high. In the case of level=0, it
>>> would mean that it is inactive.
>>>
>>> Similarly, in the case of an edge interrupt edge=1 or level=1 would mean
>>> that there is an edge, no matter if it is a rising or falling.
>>
>> Am I understanding right that you propose to fold two properties into
>> a single bit?
> 
> I don't think I understand what are the two properties that my proposal
> is merging into a single bit.
> 
> The hypercall specifies the state of the line in terms of "high" and
> "low". My proposal is to replace it with "fire the interrupt" for edge
> interrupts, and "interrupt enabled/disabled" for level, abstracting away
> the state of the line in terms of high/low and instead focusing on
> whether the interrupt should be injected or not.

Okay, I realize I misunderstood. There's a naming issue that I think
gets in the way here: Since this is about triggering an IRQ without
"setting" its specific properties, perhaps "trigger_irq" would be a
better name, with your boolean distinguishing the "assert" and
"deassert" cases (and the other one telling "edge" vs "level")?

Jan