Re: [PATCH RFC 15/19] iommu/arm-smmu-v3: Add set_dirty_tracking_range() support

Joao Martins
2022-05-02T11:53:11Z

On 4/29/22 20:20, Robin Murphy wrote:
> On 2022-04-29 17:40, Joao Martins wrote:
>> On 4/29/22 17:11, Jason Gunthorpe wrote:
>>> On Fri, Apr 29, 2022 at 03:45:23PM +0100, Joao Martins wrote:
>>>> On 4/29/22 13:23, Jason Gunthorpe wrote:
>>>>> On Fri, Apr 29, 2022 at 01:06:06PM +0100, Joao Martins wrote:
>>>>>
>>>>>>> TBH I'd be inclined to just enable DBM unconditionally in
>>>>>>> arm_smmu_domain_finalise() if the SMMU supports it. Trying to toggle it
>>>>>>> dynamically (especially on a live domain) seems more trouble that it's
>>>>>>> worth.
>>>>>>
>>>>>> Hmmm, but then it would strip userland/VMM from any sort of control (contrary
>>>>>> to what we can do on the CPU/KVM side). e.g. the first time you do
>>>>>> GET_DIRTY_IOVA it would return all dirtied IOVAs since the beginning
>>>>>> of guest time, as opposed to those only after you enabled dirty-tracking.
>>>>>
>>>>> It just means that on SMMU the start tracking op clears all the dirty
>>>>> bits.
>>>>>
>>>> Hmm, OK. But aren't really picking a poison here? On ARM it's the difference
>>>> from switching the setting the DBM bit and put the IOPTE as writeable-clean (which
>>>> is clearing another bit) versus read-and-clear-when-dirty-track-start which means
>>>> we need to re-walk the pagetables to clear one bit.
>>>
>>> Yes, I don't think a iopte walk is avoidable?
>>>
>> Correct -- exactly why I am still more learning towards enable DBM bit only at start
>> versus enabling DBM at domain-creation while clearing dirty at start.
>
> I'd say it's largely down to whether you want the bother of
> communicating a dynamic behaviour change into io-pgtable. The big
> advantage of having it just use DBM all the time is that you don't have
> to do that, and the "start tracking" operation is then nothing more than
> a normal "read and clear" operation but ignoring the read result.
>
> At this point I'd much rather opt for simplicity, and leave the fancier
> stuff to revisit later if and when somebody does demonstrate a
> significant overhead from using DBM when not strictly needed.
> OK -- I did get the code simplicity part[*]. Albeit my concern is that last
point: if there's anything fundamentally affecting DMA performance then
any SMMU user would see it even if they don't care at all about DBM (i.e. regular
baremetal/non-vm iommu usage).

[*] It was how I had this initially PoC-ed. And really all IOMMU drivers dirty tracking
could be simplified to be always-enabled, and start/stop is essentially flushing/clearing
dirties. Albeit I like that this is only really used (by hardware) when needed and any
other DMA user isn't affected.