[libvirt] One question about ACS for pci passthrough

[Daniel P. Berrange]

On Mon, Sep 16, 2013 at 04:13:01PM +0800, Li Zhang wrote:
> Hi all,
> 
> In libvirt, it doesn't allow non-ACS switch to assign devices to guests.
> Is this required for X86 to support PCI passthrough?
> 
> On my power machine, ACS capability can't be found. I think HW
> doesn't support it.
> I don't know whether this is a special situation, and I have to set
> relaxed_acs_check = 1.

If the switch supports ACS, then libvirt knows for certain whether it
is safe to assign devices to the guest. In the case where a switch
does not support ACS, there is no way for libvirt to determine whether
it is safe to assign individual downstream PCI devices to the guest.

For the sake of safety, libvirt will thus refuse to assign devices
behind a switch lacking ACS, unless you override it with the config
setting you mention.

I am not sure how this works in a VFIO world though - perhaps the
kernel VFIO grouping code has the same restriction ? I'm not sure.

Daniel
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