From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dante Cinco <dantecinco@gmail.com>
Subject: swiotlb=force in Konrad's xen-pcifront-0.8.2 pvops domU
 kernel with PCI passthrough
Date: Wed, 10 Nov 2010 17:16:14 -0800
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We have Fibre Channel HBA devices that we PCI passthrough to our pvops
domU kernel. Without swiotlb=force in the domU's kernel command line,
both domU and dom0 lock up after loading the kernel module drivers for
the HBA devices. With swiotlb=force, the domU and dom0 are stable
after loading the kernel module drivers but the I/O performance is at
least an order of magnitude worse than what we were seeing with the
HVM kernel. I see the following in /var/log/kern.log in the pvops
domU:

PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
Placing 64MB software IO TLB between ffff880005800000 - ffff880009800000
software IO TLB at phys 0x5800000 - 0x9800000

Is swiotlb=force responsible for the I/O performance degradation? I
don't understand what swiotlb=force does so I would appreciate an
explanation or a pointer.

Thanks.

- Dante