Re: [Xen-devel] question about io path in the front/backend

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

>   I have read some documents and wiki about split driver in xen,and I am
> confused about the I/O path ,in which a sys_read() pass through the domU
> and dom0,does sys_read() in the domU pass through vfs and ,say ,ext3fs
> in domU,and insert request into the requeest_queue of the
> frontend-driver,is it right?

Sounds like you have the right idea.  Requests get queued with the frontend 
driver in terms of Linux structures.  IO requests to satisfy these are then 
placed into the shared memory ring so that the backend can find out what 
we're asking for.

>   and then ,say domU sets up with a *.img file in the dom0, then what
> does frontend and backend driver do?
> does frontend transmit the request to the backend ,is it right? 

Yes, the frontend does this by putting requests into the shared memory 
ringbuffer which is also accessible by the backend.  The frontend then sends 
and event to the backend; this causes an interrupt in the backend so that it 
knows it must check the shared memory.

>   and then what does backend driver do ? does backend transfer the
> request to the phyiscal driver in the dom0 ,is it right?

Yes.  The backend responds to the interrupt by checking the shared memory for 
new requests, then it maps parts of the domUs memory so that dom0 will be 
able to write data into it.  Then it submits requests to the Linux block IO 
subsystem to fill that memory with data.  The Linux block IO system 
eventually sends these requests to the device driver, to do the IO directly 
into the mapped domU memory.

>  or does backend transfer the request into some read()operation ,and
> submit it to the vfs and ,say,ext3fs in dom0, and do another relatively
> complete io path in the dom0,is it right?

If you're just exporting a phy: device to the guest, then the block IO 
requests go down to the block device driver for that device and are serviced 
there.  e.g. if I export IDE driver phy:/dev/hda to my guest, then the IDE 
driver will satisfy the IO requests directly.
Requests go backend -> block layer -> real device driver

If you're using a file: device then you have to go through the filesystem 
layer...  So the IO requests go backend -> block layer -> loopback block 
device -> ext3 -> block layer (again) -> real device driver

If you're using blktap then the requests take a trip via userspace before 
getting submitted.

>  or  if  backend  transfer the request to physical driver directly, how
> does the backend deal with the request's virtual address ,and how does
> backend manage bio buffer ,does physical driver and backend and frontend
> share the bio buffer in  some way, or what does xen deal with it ?

I hope what I've said clarifies things a bit.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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