Re: [PATCH RFC] pass write value to in_range pointers

Gregory Haskins
2009-06-23T11:41:20Z

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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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