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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-10-24 11:45, Avi Kivity wrote:
> On 10/21/2011 11:19 AM, 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 itself, this does not provide enough value to offset the cost of a
> new ABI, especially as userspace will need to continue supporting the
> old method for a very long while.

Yes, but less sophistically as it would now.

> 
>> 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.
>>
> 
> With the new feature it may be worthwhile, but I'd like to see the whole
> thing, with numbers attached.

It's not a performance issue, it's a resource limitation issue: With the
new API we can stop worrying about user space device models consuming
limited IRQ routes of the KVM subsystem.

Jan

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