Date: Thu, 1 Jul 2010 19:22:45 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Tom Lyon <pugs@cisco.com>, randy.dunlap@oracle.com,
       linux-kernel@vger.kernel.org, kvm@vger.kernel.org, chrisw@sous-sol.org,
       joro@8bytes.org, hjk@linutronix.de, avi@redhat.com, gregkh@suse.de,
       aafabbri@cisco.com, scofeldm@cisco.com,
       Donald Dutile <ddutile@redhat.com>
Subject: Re: [PATCH V2] VFIO driver: Non-privileged user level PCI drivers
Message-ID: <20100701162245.GA2641@redhat.com>
References: <4c0eb470.1HMjondO00NIvFM6%pugs@cisco.com>
 <1277998144.10112.58.camel@x201>
 <20100701153115.GA2008@redhat.com>
 <1277999321.10112.63.camel@x201>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1277999321.10112.63.camel@x201>
User-Agent: Mutt/1.5.20 (2009-12-10)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2544
Lines: 61

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/