Re: [PATCH v4 1/2] iommu/arm-smmu-v3: Add boolean bypass_ste and skip_cdtab flags

[Jason Gunthorpe]

On Wed, Sep 20, 2023 at 01:52:03PM -0700, Nicolin Chen wrote:
> If a master has only a default substream, it can skip CD/translation table
> allocations when being attached to an IDENTITY domain, by simply setting
> STE to the "bypass" mode (STE.Config[2:0] == 0b100).
> 
> If a master has multiple substreams, it will still need a CD table for the
> non-default substreams when being attached to an IDENTITY domain, in which
> case the STE.Config is set to the "stage-1 translate" mode while STE.S1DSS
> field instead is set to the "bypass" mode (STE.S1DSS[1:0] == 0b01).
> 
> If a master is attached to a stage-2 domain, it does not need a CD table,
> while the STE.Config is set to the "stage-2 translate" mode.
> 
> Add boolean bypass_ste and skip_cdtab flags in arm_smmu_attach_dev(), to
> handle clearly the cases above, which also corrects the conditions at the
> ats_enabled setting and arm_smmu_alloc_cd_tables() callback to cover the
> second use case.
> 
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 35 ++++++++++++++++-----
>  1 file changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index df6409017127..dbe11997b4b9 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -2381,6 +2381,7 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
>  	struct arm_smmu_device *smmu;
>  	struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
>  	struct arm_smmu_master *master;
> +	bool byapss_ste, skip_cdtab;
>  
>  	if (!fwspec)
>  		return -ENOENT;
> @@ -2416,6 +2417,24 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
>  
>  	master->domain = smmu_domain;
>  
> +	/*
> +	 * When master attaches ARM_SMMU_DOMAIN_BYPASS to its single substream,
> +	 * set STE.Config to "bypass" and skip a CD table allocation. Otherwise,
> +	 * set STE.Config to "stage-1 translate" and allocate a CD table for its
> +	 * multiple stage-1 substream support, unless with a stage-2 domain in
> +	 * which case set STE.config to "stage-2 translate" and skip a CD table.
> +	 */

It might be clearer like this:

static bool arm_smmu_domain_needs_cdtab(struct arm_smmu_domain *smmu_domain,
					struct arm_smmu_master *master)
{
	switch (smmu_domain->stage) {
	/*
         * The SMMU can support IOMMU_DOMAIN_IDENTITY either by programming
         * STE.Config to 0b100 (bypass) or by configuring STE.Config to 0b101
         * (S1 translate) and setting STE.S1DSS[1:0] to 0b01 "bypass". The
         * latter requires allocating a CD table.
	 *
	 * The 0b100 config has the drawback that ATS and PASID cannot be used,
         * however it could be higher performance. Select the "S1 translation"
         * option if we might need those features.
	 */
	case ARM_SMMU_DOMAIN_BYPASS:
		return master->ssid_bits || arm_smmu_ats_supported(master);
	case ARM_SMMU_DOMAIN_S1:
	case ARM_SMMU_DOMAIN_NESTED:
		return true;
	case ARM_SMMU_DOMAIN_S2:
		return false;
	}
	return false;
}

Then the below is

       if (needs_cdtab || smm_domain->stage != ARM_SMMU_DOMAIN_BYPASS)
		master->ats_enabled = arm_smmu_ats_supported(master);

And the CD table should be sync'd to the result of arm_smmu_domain_needs_cdtab()..

It looks like there is still some kind of logic missing as we need to
know if there are any PASIDs using the cd table here:

if (!master->cd_table_empty && !needs_cdtab)
   return -EBUSY;

if (needs_ctab && !master->cd_table.cdtab)
     ret = arm_smmu_alloc_cd_tables(master);

if (!needs_ctab && master->cd_table.cdtab)
    arm_smmu_dealloc_cd_tables(master);

And add master->cd_table_emty to the arm_smmu_domain_needs_cdtab bypass logic.

Also, are these patches are out of order, this should come last since
the arm_smmu_write_strtab_ent hasn't learned yet how to do
STRTAB_STE_1_S1DSS_BYPASS?

Jason