Good brainer. ... I am geting back to few basics.
I will leave HVM and emulation aside :)
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 ? )
2. PCI-passthrough enables domU to own the PCI device. Now dom0 can no
longer use that device .
If domU has mapped address space of the required PCI device , why
need it talk to dom0 any further ?
What kind of hand shake is involved ( setup and tear down as you
have mentioned ) between domU and
domU " in passthrough case" ?
3. DomU discovers all PCI devices through Xenstore.
Mark Williamson wrote:
I have been trying to understand PCI -passthrough support. Please
correct me if I am wrong in my following inferences.
1. Device emulation and pass through are both implemented using split
drivers.
I'm going to be pedantic now ;-)
Device "emulation" is really what we do for fully virtualised (HVM) guests:
the device models provided by QEmu emulate real world devices in terms of
their responses to particular port IOs, mmapped IO operations, etc. This
isn't done using a front / back model because the guest is just using it's
normal drivers for the "real" devices.
The virtual devices used by the PV drivers are implemented using split
drivers, though, as you say.
The PCI passthrough support for PV guest is also implemented using a split
driver that implements the functions of the PCI bus in order to give the
guest the information it
Which is this information we are referring to here ?
requires to talk to its assigned device(s).
How are devices "assigned" to domU ? I am specifically talking about
late binding.
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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