Re: [Xen-devel] Other PCI devices to mark mark as read-only for dom0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.06.12 at 12:08, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 22/06/12 10:43, Jan Beulich wrote:
>> Perhaps, but that's not the only thing to deal with here. As
>> said previously, when we want to add devices with active BARs
>> here (luckily Wei confirmed that AMD IOMMUs have none),
>> Dom0 trying to re-configure them would get us into problems.
>> The issue exists today, but could become worse when we
>> disallow the updates (as that could lead to two devices sharing
>> resources they shouldn't share, whereas today a device in use
>> by Xen and getting re-assigned its resources would merely stop
>> working).
> 
> It is certainly not an easy problem, and perhaps I am needlessly
> complicating the issue.
> 
> It occurs that we have 3 possible directions to fix this issue.
> 
> 1) Continue the current method of fixing things up after they break,
> which can cause a hassle for a user encountering the issue.
> 2) Mark as RO and provide an explicit hypercall interface to remap
> BARs.  I don't know how well this would go with upstream Linux.

And what would the hypercall do? You don't expect it to fix up
all the uses of the old address(es) to now use the new one(s),
do you? Leaving aside the difficulty in getting this right, in some
cases this might not even be possible in a seamless manner.

> 3) Extend current infrastructure to be able to tell when a write is
> affecting the BARs and permit them.  This seems like the best option
> going forward, but might be quite hard to implement.

Doing the mechanical trap-and-emulate part of this shouldn't
be that hard, namely on top of that other patch I'm about to
commit.

But the thing is that this has the same problems as the hypercall
one above.

> I guess the real question is whether we should continue reactively
> fixing problems, or start protectively fixing the root cause.

I agree with this position from a theoretical standpoint.

>  My gut
> feeling is that we are going to start seeing more and more devices on
> the PCI bus which Xen should be using, rather than dom0.

That would be bad - surfacing arbitrary devices on the PCI bus
is - in my opinion - rather questionable a design (which is also
why I consider this aspect of the AMD IOMMUs badly designed).

Jan


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