Re: virtio implementation?

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Rusty Russell wrote:
> On Tue, 2007-07-17 at 23:49 -0700, Dor Laor wrote:
>   
>>> On Tue, 2007-07-17 at 06:31 -0700, Dor Laor wrote:
>>>       
>>>> btw: Rusty - what do you think of virtio for the host?
>>>>         
>>> You mean backend?  For networking it makes a great deal of sense.  For
>>> block it makes far less sense (COW, weird formats, etc).
>>>       
>> I meant networking, but there are more components we can add to the
>> host:
>> 	- Block device can be directed into blkTap thus making the
>> result both flexible and zero-copy.
>>     
>
> 	Let's avoid the term "zero-copy" since it usually indicates that the
> people in the discussion can't count.  

Well, for block you can do zero copy, using O_DIRECT.

> Let's further assume that kvm
> changes to the lguest model where guest pages are user pages.
>   

That's a valid assumption.  The rest of the discussion doesn't depend on
it, does it?

> 	AFAICT the only difference between a userspace host block driver and a
> kernel host block driver is now the system call overhead, and the
> difficulty of doing efficient write barriers from userspace.
>
>   

True.  For mortal people setups where you have at most a few thousand
ops/sec, the overhead is negligible.  In-kernel host drivers make sense
only for 100+ disks.  These people are better off using blockdevs rather
than file, so an in-kernel block device need not support file-based storage.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


-------------------------------------------------------------------------
This SF.net email is sponsored by DB2 Express
Download DB2 Express C - the FREE version of DB2 express and take
control of your XML. No limits. Just data. Click to get it now.
http://sourceforge.net/powerbar/db2/