Cam panic on r271170

[Ryan Stone]

On Wed, Sep 24, 2014 at 11:48 AM, Bryan Drewery <bdrewery at freebsd.org> wrote:
> Another, with much more information here:
> https://people.freebsd.org/~bdrewery/cam.panic.txt
>
> This was with memguard (vm.memguard.desc="CAM CCB") and a KASSERT in
> malloc(9) and uma_zalloc_arg() to prevent M_WAITOK in non-sleepable
> threads. Neither triggered.

Well that's unfortunate.  Are CCBs ever the target of DMA?

Does your hardware have an Intel chipset and is it new enough to
support an IOMMU?  You might try enabling the IOMMU, which would catch
out-of-bounds DMA by hardware:

http://svnweb.freebsd.org/changeset/base/257251

One caveat is that while I understand that the busdma infrastructure
for this is quite well tested, individual drivers and devices need to
be well-behaved so it's possible that this won't work on your hardware
right now.  Also, don't enable this if you use BHyve with PCI
Passthrough.