[PATCH 4/5] iommu/arm-smmu-v3: Add set_dirty_tracking() support

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Jason Gunthorpe <jgg at nvidia.com>
> Sent: Wednesday, November 29, 2023 7:43 PM
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi at huawei.com>
> Cc: iommu at lists.linux.dev; linux-arm-kernel at lists.infradead.org;
> robin.murphy at arm.com; will at kernel.org; joro at 8bytes.org;
> kevin.tian at intel.com; nicolinc at nvidia.com; mshavit at google.com;
> eric.auger at redhat.com; joao.m.martins at oracle.com; jiangkunkun
> <jiangkunkun at huawei.com>; zhukeqian <zhukeqian1 at huawei.com>;
> Linuxarm <linuxarm at huawei.com>
> Subject: Re: [PATCH 4/5] iommu/arm-smmu-v3: Add set_dirty_tracking()
> support
> 
> On Tue, Nov 28, 2023 at 09:49:39AM +0000, Shameer Kolothum wrote:
> > From: Joao Martins <joao.m.martins at oracle.com>
> >
> > 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(). But it
complicates things if we have to incorporates updating entries for any
new mappings.  And it was suggested to keep it enabled here,

https://lore.kernel.org/kvm/2d369e58-8ac0-f263-7b94-fe73917782e1@linux.intel.com/T/#m3be8a185668810fb24a030617f5eaf79a9a32748

> 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?

Thanks,
Shameer