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

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Jason Gunthorpe <jgg at nvidia.com>
> Sent: Sunday, May 12, 2024 1:09 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;
> ryan.roberts at arm.com; 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 v3 4/4] iommu/arm-smmu-v3: Enable HTTU for stage1
> with io-pgtable mapping
> 
> 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?

Our initial tests has not shown any difference so far. But it is a possibility.
 
> Should this start out as dirty and let the enable flow clean it to
> turn it on?

Ok. Just to be clear, so the suggestion is just set the DBM bit here and let the
read_and_clear_dirty() set the  AP_RDONLY_BIT through the iopt_set_dirty_tracking()
path.

> 
> Looks fine otherwise:
> 
> Reviewed-by: Jason Gunthorpe <jgg at nvidia.com>

Thanks,
Shameer