Re: [PATCH] iommu/arm-smmu-v3-sva: Enable Hardware Access and Hardware Dirty bits

[Pranjal Shrivastava <praan@google.com>]

On Sat, May 09, 2026 at 12:56:57AM -0700, Nicolin Chen wrote:
> On Fri, May 08, 2026 at 03:24:32PM +0100, Robin Murphy wrote:
> > On 2026-05-08 2:57 pm, Pranjal Shrivastava wrote:
> > > I see, so IIUC, you mean if IS_ENABLED(CONFIG_ARM64_HW_AFDBM) but CPU
> > > doesn't enable HTTU, it is perfectly safe to let the SMMU do HTT updates,
> > > Since the fault handlers are already expecting HW-triggered updates?
> > > 
> > > Which means our check would be something like:
> > > 
> > >     if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM) {
> > >     	if (smmu->features & FEAT_HA)
> > > 	 ...
> > >     }
> > > 
> > > instead of cpu_has_hw_af()?
> > 
> > Hmm, looking closer, cpu_has_hw_af() is the thing which actually influences
> > mm behaviour (via arch_has_hw_pte_young and arch_wants_old_prefaulted_pte),
> > and that can still be false at runtime if ARM64_HW_AFDBM is enabled but any
> > CPU doesn't support HAFDBS, so perhaps you were right the first time :)
> 
> IIUIC, v2 should be:
> 
> +		/*
> +		 * Enable Hardware Access and Dirty updates (DBM) if supported by
> +		 * both the SMMU and the CPU. It is unsafe to enable SMMU's HTTU,
> +		 * if the CPU does not support it as it bypasses mm page aging.
> +		 */
> +		if (cpu_has_hw_af()) {

Ack, yes. IMO, this is the correct system-wide gate.

> +			if (master->smmu->features & ARM_SMMU_FEAT_HA)
> +				target->data[0] |= cpu_to_le64(CTXDESC_CD_0_TCR_HA);
> +			if (master->smmu->features & ARM_SMMU_FEAT_HD)
> +				target->data[0] |= cpu_to_le64(CTXDESC_CD_0_TCR_HD);
> +		}
> 

Thanks,
Praan