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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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).

Jan


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