Subject: Re: [RFC 1/2] PCI-Express Non-Transparent Bridge Support

From: Jon Mason <[email-masked]>
Date: Fri, 13 Jul 2012 23:19:54 -0700

On Fri, Jul 13, 2012 at 05:13:44PM -0700, Stephen Hemminger wrote:
> On Fri, 13 Jul 2012 14:44:59 -0700
> Jon Mason <[email-masked]> wrote:
> 
> > A PCI-Express non-transparent bridge (NTB) is a point-to-point PCIe bus
> > connecting 2 systems, providing electrical isolation between the two subsystems.
> > A non-transparent bridge is functionally similar to a transparent bridge except
> > that both sides of the bridge have their own independent address domains.  The
> > host on one side of the bridge will not have the visibility of the complete
> > memory or I/O space on the other side of the bridge.  To communicate across the
> > non-transparent bridge, each NTB endpoint has one (or more) apertures exposed to
> > the local system.  Writes to these apertures are mirrored to memory on the
> > remote system.  Communications can also occur through the use of doorbell
> > registers that initiate interrupts to the alternate domain, and scratch-pad
> > registers accessible from both sides.
> > 
> > The NTB device driver is needed to configure these memory windows, doorbell, and
> > scratch-pad registers as well as use them in such a way as they can be turned
> > into a viable communication channel to the remote system.  ntb_hw.[ch]
> > determines the usage model (NTB to NTB or NTB to Root Port) and abstracts away
> > the underlying hardware to provide access and a common interface to the doorbell
> > registers, scratch pads, and memory windows.  These hardware interfaces are
> > exported so that other, non-mainlined kernel drivers can access these.
> > ntb_transport.[ch] also uses the exported interfaces in ntb_hw.[ch] to setup a
> > communication channel(s) and provide a reliable way of transferring data from
> > one side to the other, which it then exports so that "client" drivers can access
> > them.  These client drivers are used to provide a standard kernel interface
> > (i.e., Ethernet device) to NTB, such that Linux can transfer data from one
> > system to the other in a standard way.
> > 
> > Signed-off-by: Jon Mason <[email-masked]>
> 
> This driver does some reimplementing of standard type operations is this
> because you are trying to use the same code on multiple platforms?
> 
> Example:
> +
> +static void ntb_list_add_head(spinlock_t *lock, struct list_head *entry,
> +			      struct list_head *list)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(lock, flags);
> +	list_add(entry, list);
> +	spin_unlock_irqrestore(lock, flags);
> +}
> +
> +static void ntb_list_add_tail(spinlock_t *lock, struct list_head *entry,
> +			      struct list_head *list)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(lock, flags);
> +	list_add_tail(entry, list);
> +	spin_unlock_irqrestore(lock, flags);
> +}
> 
> Which are used on skb's and yet we already have sk_buff_head with locking?
> 
> I know you probably are committed to this API, but is there some way to
> reuse existing shared memory used by virtio-net between two ports?
> 
> 
The intention is to be able to have multiple client drivers/virtual devices that are able to use NTB as the transport to the remote system.  This is the reason why a void* is passed into the transport instead of skb*, making all of the extra book keeping necessary.  Currently, only the virtual Ethernet has been done, which may be part of the confusion.  I'd like to be able to find a way to have the virtio devices use ntb (and save me the work of reinventing the wheel), but step one is getting this code accepted :)

Thanks,
Jon