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

[Sheng Yang <sheng@linux.intel.com>]

On Thursday 16 July 2009 20:31:01 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

One comments for shared interrupt: if you means guest device shares interrupt 
with device in other domain(that means guest or host), it's still a security 
hole, and our position seems still won't-do it. Could you explain how the 
situation change with this patch? I am not sure if I understand your meaning 
completely...

Thanks.

-- 
regards
Yang, Sheng

> - 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
>
> >  Will it conflict?
>
> No in a sense that you can't bind both drivers to the same device.
>
> > In fact, it looks like you copied the comments for this driver directly
> > from the pci-stub driver :)
>
> Right.
>
> > How about moving that documentation into a place that people will notice
> > it, like the rest of the UIO documentation?
>
> OK.
>
> > And right now you are just sending the irq to userspace, what is
> > userspace supposed to do with it?
>
> Userspace uses libpci (i.e. pci sysfs) to talk to the device and to
> re-enable interrupts by writing to the command register.
>
> In the case of device assignment, this will be qemu which
> acts as a proxy for driver running in guest context.
> In case of uio loaded in guest, the driver will be in guest
> userspace, and the device is emulated in qemu.
>
> > Do you have a userspace program that
> > uses this interface today to verify that everything works?  If so, care
> > to provide a pointer to it?
>
> Sure. I used an emulated device for this.
> First, you patch qemu to add the device:
> http://www.linux-kvm.org/downloads/mst/test_irq.patch
>
> Now, run with the new kernel (-kernel flag), adding
>  -device test-irq
>
> Once in guest, assign the device id
> echo "1af4 2009" > /sys/bus/pci/drivers/uio_pci_generic/new_id
>
> Compile and run this driver:
> http://www.linux-kvm.org/downloads/mst/testuio.c
> (it does not use any libraries besides libc,
>  so just gcc testuio.c -o testuio)
>
> And now make the device assert interrupts, like this:
>
> while
> sleep 1
> do
> setpci -s 00:04.0 0x40.B=0x1
> done
>
> You should see messages printed as the driver gets interrupts, but no
> error messages about missed interrupts.
>
> > thanks,
> >
> > greg k-h
>
> --
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