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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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@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.

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.