Re: paravirtual drivers?

[Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Arjan van de Ven wrote:
> On Thu, 2007-04-12 at 10:05 -0500, Anthony Liguori wrote:
>   
>> Why not drive the backend from userspace? 
>>     
>
> (nothing in the concept requires it to be in the kernel, it just makes
> things a lot easier)
>
> Because I think the trend forward is to reduce the userspace dependency
> of kvm, not increase it.

There's no increase in dependency since block IO is already done in 
userspace.  Besides, I think the general Linux trend of moving things to 
userspace when there's no compelling reason to have them in kernel space 
trumps all other trends ;-)

>  And because you add context switches etc etc
>   

There's no context switch just a privilege transition.  Granted, 
privilege transitions aren't free but they're aren't that much compared 
to the cost of processing an IO request.

>>  Are you using a ring queue to 
>> transmit bio's from the guest to the host? 
>>     
>
> sort of; it's not quite a real ring (I'm using the block layer tagged
> queueing which is more efficient than a ring actually)
>   

I'm not sure I understand.  I guess I can just read the code.  What I 
was wondering was whether you put the bio's into a shared ring queue or 
whether you issued a hypercall per bio.

>>  What are you using for 
>> discovery (I assume PCI)?
>>     
>
> I just added a hypercall to enumerate. Adding a dummy PCI is probably
> needed for OS installers to auto-load the drivers.
>   

Ah, okay.

>> Could you post a copy of your driver on the list?  I think it would be 
>> good to discuss.
>>     
>
> I'll dig it up
>   

Excellent.  Thank you!

Regards,

Anthony Liguori


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