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

["Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>]

Acked-by: Xiantao Zhang <xiantao.zhang@xxxxxxxxx>
Xiantao
-----Original Message-----
From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx] 
Sent: Friday, December 13, 2013 11:44 PM
To: Jan Beulich
Cc: George Dunlap; Zhang, Xiantao; xen-devel; Keir Fraser; Tim Deegan
Subject: Re: [Xen-devel] [PATCH v2 1/5] IOMMU: make page table population 
preemptible

On 13/12/2013 15:41, Jan Beulich wrote:
>>>> 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
>

All true.

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel