Re: [Xen-devel] heavy P2M lock contention on guest HPET counter reads

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.08.14 at 04:33, <andres@xxxxxxxxxxxxxxxx> wrote:
> On Fri, Aug 1, 2014 at 2:38 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> 
>> >>> On 30.07.14 at 20:15, <andres@xxxxxxxxxxxxxxxx> wrote:
>> > Having said that, I don't know off the top of my head that is obviously
>> > correct to shortcut the p2m lookup for msix table and iommu. I think so,
>> > but...
>>
>> All internal MMIO ranges are what you'd call "positively decoded" on
>> real hardware, i.e. they ought to supersede any other address
>> decoding rules anyway. Since they're also not associated with
>> possibly varying MFNs, the P2M lookup on them is simply pointless,
>> and short-circuiting them is imo at once making better guarantees
>> towards consistent behavior.
>>
> Ok. Good. However: this is a very hot path. I wonder if this iteration can
> be made more efficient (i.e sort the resulting mmio regions and do binary
> search).

How much more efficient would a binary search be for currently five
handlers? Furthermore, ->check_handler() involves more than just
an address range comparison in at least some of the cases.

Jan


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