linux-kernel - Re: [PATCH v1 06/14] iommu/arm-smmu-v3: Unset corresponding STE fields when s2

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Date:   Thu, 9 Mar 2023 13:13:11 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Nicolin Chen <nicolinc@...dia.com>, jgg@...dia.com, will@...nel.org
Cc:     eric.auger@...hat.com, kevin.tian@...el.com,
        baolu.lu@...ux.intel.com, joro@...tes.org,
        shameerali.kolothum.thodi@...wei.com, jean-philippe@...aro.org,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 06/14] iommu/arm-smmu-v3: Unset corresponding STE
 fields when s2_cfg is NULL

On 2023-03-09 10:53, Nicolin Chen wrote:
> From: Eric Auger <eric.auger@...hat.com>
> 
> Despite the spec does not seem to mention this, on some implementations,
> when the STE configuration switches from an S1+S2 cfg to an S1 only one,
> a C_BAD_STE error would happen if dst[3] (S2TTB) is not reset.

Can you provide more details, since it's not clear whether this is a 
hardware erratum workaround or a bodge around the driver itself doing 
something wrong like not doing a proper break-before-make transition of 
the STE. The architecture explicitly states that all the STE.S2* fields 
except S2VMID and potentially S2S are ignored when Stage 2 is bypassed.

Thanks,
Robin.

> Explicitly reset those two higher 64b fields, to prevent that.
> 
> Note that this is not a bug at this moment, since a 2-stage translation
> setup is not yet enabled, until the following patches add its support.
> 
> Reported-by: Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> 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 c5616145e2a3..29e36448d23b 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -1361,6 +1361,9 @@ static void arm_smmu_write_strtab_ent(struct arm_smmu_master *master, u32 sid,
>   		dst[3] = cpu_to_le64(s2_cfg->vttbr & STRTAB_STE_3_S2TTB_MASK);
>   
>   		val |= FIELD_PREP(STRTAB_STE_0_CFG, STRTAB_STE_0_CFG_S2_TRANS);
> +	}  else {
> +		dst[2] = 0;
> +		dst[3] = 0;
>   	}
>   
>   	if (master->ats_enabled)