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Re: [libvirt-users] PCI passthrough and abstraction

On 4/25/19 10:14 AM, Mauricio Tavares wrote:
So I am reading through , and am wondering what is the difference between

<hostdev mode='subsystem' type='pci' managed='yes'>


<interface type='hostdev' managed='yes'>

if I am trying to give full access to a NIC? Which one exposes more of the card?

I also answered this on IRC, but just in case someone is looking through the email archives and comes across this message:

There are a couple of differences between <hostdev> and <interface type='hostdev'>:

1) <hostdev> will work with "many" different devices (as long as they are a PCI endpoint device, and don't share an IOMMU group with other devices that must remain in use by the host), but <interface type='hostdev'> can only be used for a PCI device that is a Virtual Function (VF) of an SR-IOV capable network card (if you're unsure whether or not your device qualifies, then it almost definitely *doesn't*

2) In preparation for using VFIO device assignment to assign the device to a guest, both types of device will be automatically unbound from their host driver and bound to the vfio-pci driver *if "managed='yes'" is set*. In addition, <interface type='hostdev'> will set the MAC address (and optionally the vlan tag) on the VF device before assigning it to the guest. This isn't done when you use <hostdev>, the result being that your guest will end up with a network device that has a random MAC address that is different each time it is started.

So if you are assigning VFs from an SR-IOV netcard, then you *really* want to use <interface type='hostdev'>. For all other cases of PCI device assignment (including netcards that aren't SR-IOV VFs), you *must* use <hostdev>.

As for "managed='yes'" - if you are never using the device directly on the host, it's highly recommended that you bind the device to the vfio-pci driver during the host bootup, and set "managed='no'" in the XML - probably 80% or more of the bugs we encounter with device assignment happen when we're unbinding the device from the host net (or whatever) driver, and binding it to vfio-pci or vice-versa - all sorts of timing issues that are essentially out of our control.

On Wed, Apr 24, 2019 at 1:13 PM Mauricio Tavares <raubvogel gmail com> wrote:

When you pass a device in the pci chain (after virsh
nodedev-dettach'ing it from host) to the guest, how much is passed
without being emulated/abstracted?

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