Re: [Xen-devel] Re: [Xen-users] Passthrough support ?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> 1. When PCI Passthrough support  'is not enabled' , how does domU access
> PCI device ?
>     I suppose they continue to communicate using pcifront/back split
> drivers and now dom0's
>     drivers are used ( right ? )

If you don't have PCI passthrough (as provided by pcifront/back) then the domU 
can't really have direct access to a PCI device.  The only accesses it'll be 
able to make are by using the virtual block / net interfaces to request dom0 
do some IO for it.

> 2. PCI-passthrough enables domU to own the PCI device. Now dom0 can no
> longer use that device .

Yep.

>     If domU has mapped address space of the required  PCI device , why
> need it talk to dom0 any  further ?

DomU owns the PCI device, but dom0 owns the PCI *bus* hardware.  To query (and 
potentially set) information in "PCI configuration space", the domU will have 
to talk to dom0 somehow, because that information comes from the PCI bus 
hardware, not from the device itself.  The PCI config space information 
includes the location of the mmio and port io regions used by the device, so 
without this communication the domU wouldn't know how to talk to its device.

Come to that, the PCI config space information is needed to determine what the 
device *is* so the domU knows what driver to use to talk to it ;-)

The PCI bus control hardware can only be owned by dom0, so it is necessary to 
indirect these operations through dom0.  pciback doesn't necessarily pass 
through config space data unmodified to pcifront (there used to be a number 
of different options on how it passed stuff through) and it doesn't 
necessarily allow pcifront to alter config space (for safety reasons).  But 
in the end it provides enough of the functionality of PCI config space to 
enable the domU to find and identify the PCI device(s) it owns, and load the 
appropriate drivers.

>     What kind of hand shake is involved ( setup and tear down as you
> have mentioned ) between domU and
>     domU  " in passthrough case" ?

Well, the operations on PCI config space will go through dom0.  That's the 
main thing I can think of right now, but there may be other operations too.

> 3. DomU discovers all PCI devices through  Xenstore.

I'm not actually sure on this point, but you should be able to figure it out 
with a scan through the code.  Post here if you have trouble understanding 
sections!

> >  requires to talk to its assigned device(s).
>
> How are devices "assigned"  to domU ?  I am specifically talking about
> late binding.

pciback needs to "claim" the device in dom0 so that no "real" device drivers 
in dom0 will try to use it.  We don't want the domains fighting over it ;-)  
After dom0 boot time, you can move devices to the control of pciback by 
writing values to sysfs to tell any current driver to release its hold on the 
device, then tell pciback to grab it.

Once pciback has grabbed a device, the rest of dom0 Linux doesn't know that a 
domU is using the device.  As far as Linux is concerned, the device is owned 
by pciback.

To give the domain access to the device, dom0 needs to issue some hypercalls 
giving the domU the rights to map the appropriate mmio regions, and access 
the appropriate IO regions.  It'll also need to have setup a connection 
between pcifront and pciback.  I guess it might stick something in Xenstore - 
maybe you can see what happens in the code?  Some of this will be done in 
pciback, some is probably administered in the tools code.

I'm sorry to be a bit vague.  It's a while since I looked at this code, so 
please bare in mind that I'm a little rusty ;-)

Cheers,
Mark


> >  Having
> > obtained this information, communication with the device is possible
> > directly using IO ports, memory IO regions, and DMA.
> >
> >> 2. But, in case of emulation drivers of dom0 are used  where as in case
> >> of passthrough ( as the name suggests ) native drivers in domU are used
> >> .
> >
> > For true emulation (qemu device model), a userspace process in dom0
> > handles modelling a "real" device and then issues IO using normal
> > userspace APIs. These get serviced by the dom0 kernel using the normal
> > device driver.
> >
> > For PV drivers, the frontend driver in the domU kernel issues requests
> > which are picked up by the backend driver in dom0's kernel, which then
> > issues requests into the IO stack.  Again this uses the normal device
> > driver in dom0 to talk to the actual device, it's just that the request
> > is made using a kernel-internal API rather than a userspace API (which
> > results in slightly different actions being taken).
> >
> >> 3. dom0 provides a virtual PCI device { an interface for device-OPs and
> >> status  of this virtual device} to domU and  through associated event
> >> channel domU makes
> >>    "synchronous" use of this device.
> >
> > domU uses this for control plane operations, but for most work it can
> > talk to its PCI device directly without going through dom0.
> >
> >> ===
> >> Queries:
> >>
> >> 1. What i am really not so sure about is ... passthrough case
> >>     Will there be requirement to map the address space of this PCI
> >> device in domU ?  Will the page which was being shared  so-far
> >> {xen_pci_sharedinfo}
> >>      for emulation , be "flipped"  ( transferred ) into domU ?
> >
> > xen_pci_sharedinfo - is that the page used to talk to the PCI backend
> > from pcifront?  If so, then no, that's just used for dom0-domU
> > communications.
>
> Well As I read in the xen0linux code , xen_pci_sharedinfo contains
> xen_pci_op where front end specifies which
> operation is to be done on which pci device. And then it keeps checking
> the status of this device through another flag in
> xen_pci_sharedinfo. I infer here that dom0's drivers are being used. Can
> you point me towards a code path where domU actually
> uses its own drivers having owned the PCI device ?
>
> > In order to map the address space of the PCI device directly, the domU is
> > given permissions to map the IO memory regions of that device into it's
> > page tables.  I think this is now possible to do using a grant table
> > operation...
> >
> > It is also given permission to access certain IO port ranges so that it
> > can use the device's port IO interfaces.
> >
> >>      2. Well ,
> >> Having read the code for linux (dom0,domU) I see that there are split
> >> device  drivers for PCI. (pci front and PCIback). Which are normally
> >>    communicating over xenbus.  which looks almost like other split
> >> drivers.  How exactly then passthrough enables use of domU's drivers ?
> >
> > The key thing to understand is that the pcifront / pciback is basically
> > just used for setup and teardown, not for the actual IO.  The real IO is
> > done directly by the domU without going through dom0.  For the block and
> > net drivers, *all* IO goes through dom0.
> >
> >> 3. And if passthrough support isnt provided how will communication
> >> between pcifront-pciback  be different ? ( I guess netbsd , freebsd do
> >> not have passthrough support yet )
> >
> > I'm not entirely clear what you're asking here, but I'll take a stab at
> > it:
> >
> > if pcifront (in domU) and pciback (in dom0) aren't available then
> > passthrough won't work.  The dom0 has to support the backend functions of
> > PCI passthrough and the domU has to know how to talk to it.  It's also
> > implicit that they're using the same interface version to talk to each
> > other - I'm not sure whether that's frozen stable or not.
> >
> > So *if* NetBSD lacks pciback support, it can't pass PCI devices to guests
> > that do.  Similarly, *if* it lacks pcifront support, it can't have
> > devices passed to it.
> >
> >> 4. What  restricts other domUs from accessing PCI device given to other
> >> domU via passrthrough support.
> >
> > There are some restrictions on what can be done in PCI config space to
> > prevent a guest fouling things up.  These need to be relaxed for some
> > awkward devices, though.
> >
> > For the device IO itself, domUs are only allowed to map mmio regions and
> > access io ports that are relevant to their device.  It's possible for
> > these to overlap with those for other devices, in which case you're
> > trusting the domU to be well behaved.  More crucially, though, giving a
> > domain a device with DMA capabilities is equivalent to giving it the
> > ability to subvert the entire machine.  DMA can't be sandboxed on most
> > current hardware, so if you give DMA rights to a VM it's automatically
> > just as trusted as dom0 with respect to not fooling about with other
> > domains, hardware, etc.
> >
> > Cheers,
> > Mark
>
> Thanks,
> Sanket
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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