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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/06/12 12:30, Jan Beulich wrote:
>> 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.

Currently my understanding is that dom0 has fairly free reign on the PCI
devices, meaning that if it decides to remap BARs and ends up with
conflicts, we already have a problem, especially if its a PCI device
that Xen is trying to use.

As Xen avoids using PCI devices as much as possible, it should be easier
(but doubtfully 'easy') to make Xen aware that PCI devices it is using
may move from under its feet.  ( I am just throwing ideas out here - it
might be that this is a stupid idea. )

>
>> 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

Agreed, but we now have a problem and need a solution.

>
>
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-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com




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