Re: [Xen-devel] [PATCHv9 3/4] gnttab: make the grant table lock a read-write lock

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 22/05/15 07:37, Jan Beulich wrote:
>>>> On 21.05.15 at 17:16, <david.vrabel@xxxxxxxxxx> wrote:
>> On 21/05/15 15:53, Jan Beulich wrote:
>>>>>> On 21.05.15 at 15:36, <david.vrabel@xxxxxxxxxx> wrote:
>>>> On 21/05/15 11:32, Jan Beulich wrote:
>>>>>>>> On 20.05.15 at 17:54, <david.vrabel@xxxxxxxxxx> wrote:
>>>>>> @@ -827,9 +828,11 @@ __gnttab_map_grant_ref(
>>>>>>              if ( (wrc + rdc) == 0 )
>>>>>>                  err = iommu_map_page(ld, frame, frame, IOMMUF_readable);
>>>>>>          }
>>>>>> +
>>>>>> +        double_gt_lock(lgt, rgt);
>>>>>
>>>>> unlock. And with this code path actually used (due to the bug it's
>>>>> pretty clear it didn't get exercised in your testing), how does
>>>>> performance look like? 
>>>>
>>>> I think it will be no worse than what it was before -- this path already
>>>> really sucks (mapcount() loops over 1000s of entries).  I don't care
>>>> about this path at all.
>>>
>>> It's kind of strange that you don't care about this case - afaict we're
>>> not getting there by default solely because dom0-strict mode is not
>>> currently the default (albeit arguably it should be).
>>
>> What's your point?  Are you going to refuse this series unless it also
>> optimizes dom0-strict mode?
> 
> No, I'm certainly not going to be. But this series of "I don't care"
> worry me to a certain degree (apart from making me wonder
> why, as so far I thought an as secure as possible environment is
> key for XenServer, and "iommu=dom0-strict" is certainly helping
> with that, which is also why I said that this should arguably
> become the default).

dom0-strict mode doesn't align with our PV-IOMMU plans to make BFN ==
PFN (including for grants).

David

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