Re: [PATCHv3 RFC] uio: add generic driver for PCI 2.3 devices

Michael S. Tsirkin
2009-07-15T05:01:13Z

On Tue, Jul 14, 2009 at 04:25:07PM -0700, Chris Wright wrote:
> * Michael S. Tsirkin (mst@redhat.com) wrote:
> > On Tue, Jul 14, 2009 at 11:03:38AM -0700, Chris Wright wrote:
> > > * Michael S. Tsirkin (mst@redhat.com) wrote:
> > > > - remove irqcontrol: user can enable interrupts by
> > > > writing command register directly
> > >
> > > Sorry if I gave you the impression that removing was needed.
> > > I actually think the irqcontrol was useful since it's atomic.
> >
> > What I'm saying, it is not strictly needed (user can safely write 0 in
> > that register and worst case you just get an extra interrupt). So let's
> > make the decision on what does irqcontrol do when we have a pressing
> > need for an extra kernel/user interface.
>
> OK. My concern is theoretical (as in the current design wouldn't
> trigger an issue):
>
> cmd = pread()
> ------+
> \
> cmd &= ~INTX_DISABLE +----+
> / |
> -------+ |
> pwrite(cmd) |
> -
> During this window Command Reg can change[1] and cmd is stale
>
> [1] due to irqhandler (doesn't matter in this case since it touches
> same bit). or due to some other thread updating Command Reg (also
> doesn't matter since this does not happen right now).

Right. Note that the limitation that only a single thread should touch
config space through sysfs applies to any sub-byte field. If we wanted
to make such ops atomic, pci devices would need some kind of compare and
swap ioctl.

--
MST