RE: [PATCH RFC v2 02/18] irq/dev-msi: Add support for a new DEV_MSI irq domain

From: Dey, Megha
Date: Wed Aug 05 2020 - 15:19:06 EST

Hi Jason,

> -----Original Message-----
> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Sent: Wednesday, July 22, 2020 12:59 PM
> To: Marc Zyngier <maz@xxxxxxxxxx>
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> Subject: Re: [PATCH RFC v2 02/18] irq/dev-msi: Add support for a new DEV_MSI
> irq domain
> On Wed, Jul 22, 2020 at 07:52:33PM +0100, Marc Zyngier wrote:
> > Which is exactly what platform-MSI already does. Why do we need
> > something else?
> It looks to me like all the code is around managing the
> dev->msi_domain of the devices.
> The intended use would have PCI drivers create children devices using mdev or
> virtbus and those devices wouldn't have a msi_domain from the platform. Looks
> like platform_msi_alloc_priv_data() fails immediately because dev->msi_domain
> will be NULL for these kinds of devices.
> Maybe that issue should be handled directly instead of wrappering
> platform_msi_*?
> For instance a trivial addition to the platform_msi API:
> platform_msi_assign_domain(struct_device *newly_created_virtual_device,
> struct device *physical_device);
> Which could set the msi_domain of new device using the topology of
> physical_device to deduce the correct domain?
> Then the question is how to properly create a domain within the hardware
> topology of physical_device with the correct parameters for the platform.
> Why do we need a dummy msi_domain anyhow? Can this just use
> physical_device->msi_domain directly? (I'm at my limit here of how much of this
> I remember, sorry)
> If you solve that it should solve the remapping problem too, as the
> physical_device is already assigned by the platform to a remapping irq domain if
> that is what the platform wants.

Yeah most of what you said is right. For the most part, we are simply introducing a new IRQ domain
which provides specific domain info ops for the classes of devices which want to provide custom
mask/unmask callbacks..

Also, from your other comments, I've realized the same IRQ domain can be used when interrupt
remapping is enabled/disabled.

Hence we will only have one create_dev_msi_domain which can be called by any device driver that
wants to use the dev-msi IRQ domain to alloc/free IRQs. It would be the responsibility of the device
driver to provide the correct device and update the dev->msi_domain.

> >> + parent = irq_get_default_host();
> > Really? How is it going to work once you have devices sending their
> > MSIs to two different downstream blocks? This looks rather
> > short-sighted.
> .. and fix this too, the parent domain should be derived from the topology of the
> physical_device which is originating the interrupt messages.

> > On the other hand, masking an interrupt is an irqchip operation, and
> > only concerns the irqchip level. Here, you seem to be making it an
> > end-point operation, which doesn't really make sense to me. Or is this
> > device its own interrupt controller as well? That would be extremely
> > surprising, and I'd expect some block downstream of the device to be
> > able to control the masking of the interrupt.
> These are message interrupts so they originate directly from the device and
> generally travel directly to the CPU APIC. On the wire there is no difference
> between a MSI, MSI-X and a device using the dev-msi approach.
> IIRC on Intel/AMD at least once a MSI is launched it is not maskable.
> So the model for MSI is always "mask at source". The closest mapping to the
> Linux IRQ model is to say the end device has a irqchip that encapsulates the
> ability of the device to generate the MSI in the first place.
> It looks like existing platform_msi drivers deal with "masking"
> implicitly by halting the device interrupt generation before releasing the
> interrupt and have no way for the generic irqchip layer to mask the interrupt.
> I suppose the motivation to make it explicit is related to vfio using the generic
> mask/unmask functionality?
> Explicit seems better, IMHO.

I don't think I understand this fully, ive still kept the device specific mask/unmask calls in the next
patch series, please let me know if it needs further modifications.
> Jason