Re: [PATCHv4] uio: add generic driver for PCI 2.3 devices

[Greg KH <gregkh@suse.de>]

On Thu, Jul 16, 2009 at 03:31:01PM +0300, Michael S. Tsirkin wrote:
> On Wed, Jul 15, 2009 at 03:08:29PM -0700, Greg KH wrote:
> > On Wed, Jul 15, 2009 at 11:13:40PM +0300, Michael S. Tsirkin wrote:
> > > This adds a generic uio driver that can bind to any PCI device.  First
> > > user will be virtualization where a qemu userspace process needs to give
> > > guest OS access to the device.
> > > 
> > > Interrupts are handled using the Interrupt Disable bit in the PCI command
> > > register and Interrupt Status bit in the PCI status register.  All devices
> > > compliant to PCI 2.3 (circa 2002) and all compliant PCI Express devices should
> > > support these bits.  Driver detects this support, and won't bind to devices
> > > which do not support the Interrupt Disable Bit in the command register.
> > > 
> > > It's expected that more features of interest to virtualization will be
> > > added to this driver in the future. Possibilities are: mmap for device
> > > resources, MSI/MSI-X, eventfd (to interface with kvm), iommu.
> > > 
> > > Acked-by: Chris Wright <chrisw@redhat.com>
> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > ---
> > > 
> > > Hans, Greg, please review and consider for upstream.
> > > 
> > > This is intended to solve the problem in virtualization that shared
> > > interrupts do not work with assigned devices. Earlier versions of this
> > > patch have circulated on kvm@vger.
> > 
> > How does this play with the pci-stub driver that I thought was written
> > to solve this very problem?
> 
> 
> AFAIK the problem pci stub was written to solve is simply to bind to a
> device. You then have to use another kernel module which looks the
> device up with something like pci_get_bus_and_slot to do anything
> useful. In particular, for non-shared interrupts, we can disable the
> interrupt in the apic. But this does not work well for shared
> interrupts. Thus this work.
> 
> The uio driver will be used in virtualization scenarious, a couple
> of possible ones that have been mentioned on the kvm list are:
> - device assignment (guest access to device) for simple devices with
>   shared interrupts: emulating PCI is tricky enough to better be done in
>   userspace. shared interrupt support is important as it happens
>   with real devices
> - simple communication between guest and host:
>   we create a virtual device in host, and userspace
>   driver in guest gets events and passes them on
>   to e.g. dbus. shared interrupt support is important
>   to avoid wasting irqs

Ah, ok, thanks for all of the explanation, that makes sense.

greg k-h