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SubjectRe: [PATCH V2] VFIO driver: Non-privileged user level PCI drivers
On Thu, Jul 01, 2010 at 09:48:41AM -0600, Alex Williamson wrote:
> On Thu, 2010-07-01 at 18:31 +0300, Michael S. Tsirkin wrote:
> > On Thu, Jul 01, 2010 at 09:29:04AM -0600, Alex Williamson wrote:
> > > On Tue, 2010-06-08 at 14:21 -0700, Tom Lyon wrote:
> > > > +The VFIO_DMA_MASK ioctl is used to set the maximum permissible DMA address
> > > > +(device dependent). It takes a single unsigned 64 bit integer as an argument.
> > > > +This call also has the side effect of enabling PCI bus mastership.
> > >
> > > Hi Tom,
> > >
> > > This interface doesn't make sense for the MAP_IOVA user. Especially in
> > > qemu, we have no idea what the DMA mask is for the device we're
> > > assigning. It doesn't really matter though because the guest will use
> > > bounce buffers internally once it loads the device specific drivers and
> > > discovers the DMA mask. This only seems relevant if we're using a
> > > DMA_MAP call that gets to pick the dmaaddr, so I'd propose we only make
> > > this a required call for that interface, and create a separate ioctl for
> > > actually enabling bus master. Thanks,
> > >
> > > Alex
> >
> > I expect there's no need for a separate ioctl to do this:
> > you can do this by write to the control register.
>
> Nope, vfio only allows direct writes to the memory and io space bits of
> the command register,

I don't see why's there need to protect the control register.
As far as I can see, nothing userspace does with it
can damage the host.

> all other bits are virtualized. I wonder if
> that's necessary though since we require the device to be attached to an
> iommu domain before we allow config space access.
>
> Alex
>

I don't think it's necessary. IMHO all the virtualization
tables can just be replaced with
if (pci header type == PCI_HEADER_TYPE_NORMAL)
if (addr < PCI_BASE_ADDRESS_0 + 24 &&
addr + len >= PCI_BASE_ADDRESS_0)
return -EPERM;
else /* similar thing for the bridge and cardbus
types */
Much simpler and more readable than tables full of 0xffff.
Reason this is enough is because virt drivers like qemu already
have the code to treat interrupt disable, MSI/MSIX capabilities
and BARs registers specially. custom userspace drivers simply
have no reason to touch anything besides the interrupt disable bit.

--
MST


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