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

[Julien Grall <julien.grall.oss@xxxxxxxxx>]

On Sat, 8 Aug 2020 at 10:27, Julien Grall <julien.grall.oss@xxxxxxxxx> wrote:
>
> On Fri, 7 Aug 2020 at 22:51, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> 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@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.
>
> I am not sure to understand why the single (or event multiple) device
> emulator needs to know the trigger type. The information of the

I mean trigger type configured by the OS here. Sorry for the confusion.

> trigger type of the interrupt would be described in the firmware table
> and it is expected to be the same as what the emulator expects.
>
> If the guest OS decided to configure wrongly the interrupt trigger
> type, then it may not work properly. But, from my understanding, this
> doesn't differ from the HW behavior.
>
> >
> > 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 rising or falling.
>
> TBH, I think your approach is only going to introduce more headache in
> Xen if a guest OS decides to change the trigger type.
>
> It feels much easier to just ask the emulator to let us know the level
> of the line. Then if the guest OS decides to change the trigger type,
> we only need to resample the line.
>
> Cheers,