linux-kernel - Re: [PATCH] kvm: remove in_range from kvm_io

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Date:	Tue, 23 Jun 2009 11:44:57 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	avi@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	mtosatti@...hat.com, paulmck@...ux.vnet.ibm.com, markmc@...hat.com
Subject: Re: [PATCH] kvm: remove in_range from kvm_io_device

Michael S. Tsirkin wrote:
> On Tue, Jun 23, 2009 at 11:21:53AM -0400, Gregory Haskins wrote:
>   
>> Michael S. Tsirkin wrote:
>>     
>>> Remove in_range from kvm_io_device and ask read/write callbacks, if
>>> supplied, to perform range checks internally. This allows aliasing
>>> (mostly for in-kernel virtio), as well as better error handling by
>>> making it possible to pass errors up to userspace. And it's enough to
>>> look at the diffstat to see that it's a better API anyway.
>>>
>>> While we are at it, document locking rules for kvm_io_device.
>>>   
>>>       
>> Sorry, not trying to be a PITA, but I liked your last suggestion better.  :(
>>
>> I am thinking forward to when we want to use something smarter than a
>> linear search (like rbtree/radix) for scaling the number of "devices"
>> (really, virtio-rings) that we support.
>>     
>
> in_range is broken for this anyway: you need more than a boolean
> predicate to implement rbtree/radix
>   

Yes, understood..in_range() needs to be (pardon the pun) "addressed"
;).  But getting rid of in_range() and moving the match logic into the
read()/write() verbs is potentially a step in the wrong direction if we
ever wanted to go that route.  And I'm pretty sure we do.

>   
>> The current device-count
>> target is 512, which we will begin to rapidly consume as the in-kernel
>> virtio work progresses.
>>     
>
> That's a large number. I had in mind more like 4 virtio devices, for
> starters: 1 for each virtqueue in net and block.
>   

Thats way to low.  For instance, I'll be wanting to do things like
802.1p which would be 16 virtio-rings per device (8 prio levels tx, 8
levels rx).  And thats just for one device.  I think Avi came up with an
estimate of supporting 20 devices @ 16 queues = 320, so we rounded it to
512.
>   
>>  This proposed approach forces us into a
>> potential O(256) algorithm in the hotpath (all MMIO/PIO exits will hit
>> this, not just in-kernel users).  How would you address this?
>>     
>
> Two ideas that come to mind:
> - add addr/len fields to devices, use these to speed up lookup
>   

Yep, thats what I was thinking as well.  We can have the top-level
(group) be an rbtree on addr/len, and then walk the list of items at
that address linearly using your read/write() approach.


> - add a small cache that can be scanned first
>   

Yep, I think we may want to do this anyway independent of the search alg.

> In both cases, you first do a fast lookup, ask the device whether
> it wants the transaction, then resort to linear scan if not
>   

-Greg




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