question on Bhyve

Neel Natu neelnatu at
Tue Sep 27 20:05:40 UTC 2011

Hi Julian,

On Mon, Sep 26, 2011 at 11:10 PM, Julian Elischer <julian at> wrote:
> does anyone know what is needed for a hypervisor to support PCI pass
> through?

I can speak about the bhyve implementation of pci passthru but I
suspect that other hypervisors do it similarly. BHyVe requires that
the platform support nested page tables and has an iommu to be able to
support pci passthru.

There are two parts to it.

The simpler part is "hiding" the pci device that is to be used to
passthru. In bhyve we do this by setting the tunable "pptdevs" to a
list of pci devices that we may want to assign as passthru devices.

This tunable is used by the "blackhole" pci device driver to claim
each device in "pptdevs" during probe but then reject it in attach.
The net effect of this is leave the pci device un-attached from the
host's perspective.

For e.g. in /boot/loader.conf:

pptdevs="1/0/0 2/0/0 3/0/0"

The second part is a bit more work for the hypervisor.

We assign the pci device to a guest with the following command line
option to /usr/sbin/bhyve: -s 1,passthru,3/0/0. This says to make the
pci device identified by "3/0/0" appear as a passthru pci device on
slot 1 of the virtual pci bus.

When we assign the device to the guest we need to provide it with
passthru access to the device's config space, mmio space, io space. In
the other direction we need to provide the pci device with access to
the guest's address space.

The pci config space is a bit tricky because we need to intercept
accesses to the PCI BARs and also to the MSI capability. But beyond
those registers we simply intercept config space accesses and fulfill
them by issuing PCIIOCREAD and PCIIOCWRITE ioctls on /dev/pci

Accesses to the MMIO space are straightforward and require no
hypervisor intervention after setting up the nested page tables.

Accesses to the I/O space allocated to the device are trapped by the
hypervisor and handled by the opening /dev/io and issuing IODEV_PIO
ioctl on it.

And finally dma transfers generated by the pci device into the guest's
address space are transparent to the hypervisor once it has set up the

Hope that helps.


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