Re: [RFC][PATCH] KVM: Introduce direct MSI message injection for in-kernel irqchips

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 21, 2011 at 01:51:15PM +0200, Jan Kiszka wrote:
> On 2011-10-21 13:06, Michael S. Tsirkin wrote:
> > On Fri, Oct 21, 2011 at 11:19:19AM +0200, Jan Kiszka wrote:
> >> Currently, MSI messages can only be injected to in-kernel irqchips by
> >> defining a corresponding IRQ route for each message. This is not only
> >> unhandy if the MSI messages are generated "on the fly" by user space,
> >> IRQ routes are a limited resource that user space as to manage
> >> carefully.
> >>
> >> By providing a direct injection with, we can both avoid using up limited
> >> resources and simplify the necessary steps for user land. The API
> >> already provides a channel (flags) to revoke an injected but not yet
> >> delivered message which will become important for in-kernel MSI-X vector
> >> masking support.
> >>
> >> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> > 
> > I would love to see how you envision extending this to add the masking
> > support at least at the API level, not necessarily the supporting code.
> > 
> > It would seem hard to use flags field for that since MSIX mask is per
> > device per vector, not per message.
> > Which gets us back to resource per vector which userspace has to manage
> > ...
> > 
> > interrupt remapping is also per device, so it isn't any easier
> > with this API.
> 
> Yes, we will need an additional field to associate the message with its
> source device. Could be a PCI address or a handle (like the one assigned
> devices get) returned on MSI-X kernel region setup. We will need a flag
> to declare that address/handle valid, also to tell apart platform MSI
> messages (e.g. coming from HPET on x86).

I have not thought about remapping a lot yet:
HPET interrupts are not subject to remapping?

> I see no obstacles ATM that
> prevent doing that on top of this API, do you?
> 
> Jan

For masking, I think I do. We need to maintain the pending bit
and the io notifiers in kernel, per vector.
An MSI injected with just an address/data pair, without
vector/device info, can't be masked properly.

We get back to maintaining some handle per vector, right?

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