Re: [Xen-devel] [PATCH v2 1/5] IOMMU: make page table population preemptible

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.12.13 at 16:09, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/12/2013 13:59, Jan Beulich wrote:
>> @@ -290,15 +307,14 @@ static int assign_device(struct domain *
>>                     rc);
>>      }
>>  
>> -    if ( has_arch_pdevs(d) && !need_iommu(d) )
>> + done:
>> +    if ( !has_arch_pdevs(d) && need_iommu(d) )
> 
> We now have a case where, for the first device, we could set up
> pagetables for a large domain, get an error with assignment, then tear
> them all back down.  (-EBUSY from pci_get_pdev() looks like a good
> non-fatal candidate for causing this behaviour)
> 
> I am wondering whether this is better for worse than the race condition
> where a guest couldn't use the device.  A guest could not reasonably
> expect to use a device before the toolstack is done setting it up.   A
> buggy toolstack could quite easily tie up a lot of Xen time creating and
> destroying complete iommu pagetable sets.

I don't think it's worth worrying about buggy tool stacks here -
they should simply get fixed.

Furthermore this change in operation ordering is only a (nice)
side effect, the necessary cleanup seemed easier with the
order changed. And said time window would have grown with
the added preemption handling.

Jan


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