Re: [PATCH v3 4/4] iommu/arm-smmu-v3: Enable HTTU for stage1 with io-pgtable mapping

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, May 22, 2024 at 07:19:05AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Sunday, May 12, 2024 8:09 PM
> > 
> > On Tue, Apr 30, 2024 at 02:43:08PM +0100, Shameer Kolothum wrote:
> > 
> > > diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-
> > arm.c
> > > index da6cc52859ba..20ac0e833c7b 100644
> > > --- a/drivers/iommu/io-pgtable-arm.c
> > > +++ b/drivers/iommu/io-pgtable-arm.c
> > > @@ -433,6 +433,8 @@ static arm_lpae_iopte arm_lpae_prot_to_pte(struct
> > arm_lpae_io_pgtable *data,
> > >  		pte = ARM_LPAE_PTE_nG;
> > >  		if (!(prot & IOMMU_WRITE) && (prot & IOMMU_READ))
> > >  			pte |= ARM_LPAE_PTE_AP_RDONLY;
> > > +		else if (data->iop.cfg.quirks & IO_PGTABLE_QUIRK_ARM_HD)
> > > +			pte |= ARM_LPAE_PTE_AP_WRITABLE_CLEAN;
> > 
> > This seems a bit suboptimal, it means the HTTU will be generating
> > dirty's before the tracking is turned on. As I understand it if the
> > SMMU wants to write a dirty bit it has to do an atomic RMW to memory,
> > so this would be a drag on baseline performance?
> > 
> > Should this start out as dirty and let the enable flow clean it to
> > turn it on?
> > 
> 
> this appears to be good for other vendors too?

I thought Intel and AMD both had a per-table flag to turn on tracking
and without that bit the dirties are not written back?

Jason