Re: [PATCH v4] KVM: Introduce direct MSI message injection for in-kernel irqchips

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

On 2012-04-04 13:50, Avi Kivity wrote:
> On 04/04/2012 01:48 PM, Jan Kiszka wrote:
>>>
>>> I'm not so sure anymore.  Sorry about the U turn, but remind me why?  In
>>> the long term it will be slower.
>>
>> Likely not measurably slower. If you look at a message through the arch
>> glasses, you can usually spot the destination directly, specifically if
>> a message targets a single processor - no need for hashing and table
>> lookups in the common case.
> 
> Not on x86.  The APIC ID is guest-provided.

...but is still a rather stable mapping on the physical ID.

>  In x2apic mode it can be
> quite large.

Yes, but then you can at least hash/search/cache inside that group only,
with a smaller scope.

> 
>> In contrast, the maintenance costs for the current explicit route based
>> model are significant as we see now.
>>
> 
> You mean in amount of code in userspace?  That doesn't get solved since
> we need to keep compatibility.

We do not need to track MSI origins to correlate them with routes (with
the exception of 3 special devices: vhost-based virtio, kvm device
assignment, and vfio device assignment). We emulate this centrally with
a hand full of LOC in the kvm layer, and we bypass it with the advent of
a direct injection API. Compare this to my original series that
introduced MSIRoutingCaches to cope with the current kernel API.

Jan

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