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sfid-20231214_082415_424792_F5C882E9 X-CRM114-Status: GOOD ( 26.63 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 30/11/2023 12:54, Jason Gunthorpe wrote: > On Thu, Nov 30, 2023 at 08:56:32AM +0000, Shameerali Kolothum Thodi wrote: >>> On Tue, Nov 28, 2023 at 09:49:39AM +0000, Shameer Kolothum wrote: >>>> From: Joao Martins >>>> >>>> Dirty tracking will always be enabled with DBM=1 modifier enabled >>>> by default when HD is supported. >>> >>> Is this trying to say that ARM doesn't have a per-table global enable >>> for dirty tracking but instead pre-sets the DBM bit to avoid the cost? >>> >> >> Yes. SMMUv3 has per-PTE DBM control and I think the initial RFC had >> it walking the PTEs and setting the DBM on set_dirty_tracking(). > > set_dirty_tracking doesn't have access to the necessary locking to > touch the PTEs. > This is done for free by iopt_clear_dirty_data(). set_dirty_tracking() is mostly a nop, under the assumption that dirty tracking is always enabled. >>> So on smmuv3 to enable we have to clear everything and disable >>> continues to pay a penalty since we don't go and mark all things as >>> dirty again? >> >> Yes we clear everything on enable. Sorry I didn't get the second part. >> We don't mark dirty on disable. How is that different from Intel/AMD? > > Intel/AMD have a global switch so they just turn off the tracking and > stop paying the cost. > > This approach on ARM means once the tracking is logically turned off > the HW will continue to generate memory traffic to set dirty bits on > DMAs. There is no way to back to the at-start state where their is 0 > memory traffic on DMAs. Not sure that it totally matters, but it is > worth noting someplace. > > If we do want to solve this then ARM would need iommufd to make a pass > over the page table to set for disable similar to how we have to clear > for enable. The firsts attempt at this had to be dynamic[0]. Like set_dirty_tracking_range would pass over the io pagetable and set the DBM bit to enable dirty tracking. It was suggested that we switch to an always-on mode to simplify initial bringup of the feature, and if the always-on was somehow affecting DMA performance, we would re-attempt at this dynamic mode post-mortem. With the current code structure, perhaps having set_dirty_tracking() do the DBM-enable pass we would need to move iorw_sem section to include the call to set_dirty_tracking() and smmu op set_dirty_tracking() op would walk the whole pt, without relying on iopt areas. Or we go back to iterating areas similar to [0] with a new op. [0] https://lore.kernel.org/kvm/20220428210933.3583-16-joao.m.martins@oracle.com/ _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel