linux-kernel - Re: [PATCH RFC] pass write value to in

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Date:	Tue, 23 Jun 2009 07:41:12 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Avi Kivity <avi@...hat.com>
CC:	"Michael S. Tsirkin" <mst@...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 RFC] pass write value to in_range pointers

Avi Kivity wrote:
> On 06/22/2009 07:08 PM, Michael S. Tsirkin wrote:
>> On Mon, Jun 22, 2009 at 11:45:00AM -0400, Gregory Haskins wrote:
>>   
>>> Michael S. Tsirkin wrote:
>>>     
>>>> It seems that a lot of complexity and trickiness with iosignalfd is
>>>> handling the group/item relationship, which comes about because kvm
>>>> does
>>>> not currently let a device on the bus claim a write transaction
>>>> based on the
>>>> value written.  This could be greatly simplified if the value written
>>>> was passed to the in_range check for write operation.  We could then
>>>> simply make each kvm_iosignalfd a device on the bus.
>>>>
>>>> What does everyone think of the following lightly tested patch?
>>>>
>>>>        
>>> Hi Michael,
>>>    Its interesting, but I am not convinced its necessary.  We
>>> created the
>>> group/item layout because iosignalfds are unique in that they are
>>> probably the only IO device that wants to do some kind of address
>>> aliasing.
>>>      
>>
>> We actually already have aliasing: is_write flag is used for this
>> purpose. Actually, it's possible to remove is_write by passing
>> a null pointer in write_val for reads. I like this a bit less as
>> the code generated is less compact ... Avi, what do you think?
>>    
>
> Greg, won't Michael's patch eliminate a big chunk from your iosignalfd
> patches?  Seems like a win to me.

Well, it really just moves that hunk from eventfd.c to kvm_main.c, where
I don't think anyone else will use it by iosignalfd.  But if that is
what everyone wants, I guess I have no choice.

>
>> One is enough :)
>> Seriously, do you see that this saves you all of RCU, linked lists and
>> counters? You don't need to keep track of iofds, you don't need to
>> implement your own lookup logic - you just use the kvm device
>> and that's it.
>>
>>    
>
> Yup.
>



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