linux-kernel - Re: [PATCH V2] VFIO driver: Non-privileged user level PCI drivers

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Date:	Thu, 1 Jul 2010 19:22:45 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Tom Lyon <pugs@...co.com>, randy.dunlap@...cle.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	chrisw@...s-sol.org, joro@...tes.org, hjk@...utronix.de,
	avi@...hat.com, gregkh@...e.de, aafabbri@...co.com,
	scofeldm@...co.com, Donald Dutile <ddutile@...hat.com>
Subject: Re: [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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