Re: pre Sandy bridge IOMMU support (gm45)

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Mon, Feb 01, 2016 at 12:59:00AM -0700, Jan Beulich wrote:
> >>> On 30.01.16 at 02:47, <marmarek@invisiblethingslab.com> wrote:
> > On Tue, Jan 26, 2016 at 04:37:05AM -0700, Jan Beulich wrote:
> >> (re-adding xen-devel)
> >> 
> >> >>> On 26.01.16 at 12:28, <thierry.laurion@gmail.com> wrote:
> >> > Iommu=0 let the whole Qubes system work, without enforcing hardware
> >> > compartimentalisation (iommu is enforced in software mode)
> >> > 
> >> > When iommu=no-igfx is enforced, shell console boot up works flawlessly. All
> >> > domu machines get booted up. A system hang will happen at the moment a domu
> >> > machine does graphic rendering,
> >> 
> >> And this is (other than I originally implied) without passing through
> >> the IGD to the DomU? If so, I can't see the difference between a
> >> guest rendering to its display (and vncviewer or whatever frontend
> >> you use converting this to rendering on the host) and rendering
> >> which originates in the host.
> > 
> > Not sure if relevant, but window content is mapped from PV domU directly
> > into X server (in dom0) address space, using xc_map_foreign_pages. It is
> > done by hacking XShmAttach function. Not sure what graphics driver do
> > with it next. Theoretically it could be possible that driver will direct IGD
> > to do DMA directly from that place, but I guess it does not.
> 
> Interesting. This then really needs to be investigated from the
> Qubes end rather than here. Possible resulting patches, if
> relevant outside of that unusual setup, would then of course be
> appreciated to be sent here.

Note that Thierry said "The point is the iommu=no-igfx doesn't fix the
issue", so either it is totally unrelated issue, or iommu=no-igfx is
broken. Does iommu=no-igfx have any meaning when IDG is _not_ passed
through to some domU? And generally - is IOMMU used in any way for dom0
devices? Is Linux kernel able to utilize it for its own purposes (my
guess: no)?

Somehow unrelated: I've tried to get p2m iommu mapping using `xl
debug-key o`, but it's too long (and xenconsoled isn't fast enough to
catch it into hypervisor.log). Is is possible to enlarge console ring
buffer at runtime? If not, is `conring_size` the right option? How large
it should be (aprox) for `xl debug-key o` output? 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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