linux-kernel - Re: [PATCH v2 1/3] iommu/arm-smmu: Add support for driver IOMMU fault handlers

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Date:   Fri, 22 Jan 2021 12:41:26 +0000
From:   Will Deacon <will@...nel.org>
To:     Jordan Crouse <jcrouse@...eaurora.org>
Cc:     linux-arm-msm@...r.kernel.org, Robin Murphy <robin.murphy@....com>,
        iommu@...ts.linux-foundation.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Krishna Reddy <vdumpa@...dia.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] iommu/arm-smmu: Add support for driver IOMMU
 fault handlers

On Tue, Nov 24, 2020 at 12:15:58PM -0700, Jordan Crouse wrote:
> Call report_iommu_fault() to allow upper-level drivers to register their
> own fault handlers.
> 
> Signed-off-by: Jordan Crouse <jcrouse@...eaurora.org>
> ---
> 
>  drivers/iommu/arm/arm-smmu/arm-smmu.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> index 0f28a8614da3..7fd18bbda8f5 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> @@ -427,6 +427,7 @@ static irqreturn_t arm_smmu_context_fault(int irq, void *dev)
>  	struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
>  	struct arm_smmu_device *smmu = smmu_domain->smmu;
>  	int idx = smmu_domain->cfg.cbndx;
> +	int ret;
>  
>  	fsr = arm_smmu_cb_read(smmu, idx, ARM_SMMU_CB_FSR);
>  	if (!(fsr & ARM_SMMU_FSR_FAULT))
> @@ -436,11 +437,20 @@ static irqreturn_t arm_smmu_context_fault(int irq, void *dev)
>  	iova = arm_smmu_cb_readq(smmu, idx, ARM_SMMU_CB_FAR);
>  	cbfrsynra = arm_smmu_gr1_read(smmu, ARM_SMMU_GR1_CBFRSYNRA(idx));
>  
> -	dev_err_ratelimited(smmu->dev,
> -	"Unhandled context fault: fsr=0x%x, iova=0x%08lx, fsynr=0x%x, cbfrsynra=0x%x, cb=%d\n",
> +	ret = report_iommu_fault(domain, dev, iova,
> +		fsynr & ARM_SMMU_FSYNR0_WNR ? IOMMU_FAULT_WRITE : IOMMU_FAULT_READ);
> +
> +	if (ret == -ENOSYS)
> +		dev_err_ratelimited(smmu->dev,
> +		"Unhandled context fault: fsr=0x%x, iova=0x%08lx, fsynr=0x%x, cbfrsynra=0x%x, cb=%d\n",
>  			    fsr, iova, fsynr, cbfrsynra, idx);
>  
> -	arm_smmu_cb_write(smmu, idx, ARM_SMMU_CB_FSR, fsr);
> +	/*
> +	 * If the iommu fault returns an error (except -ENOSYS) then assume that
> +	 * they will handle resuming on their own
> +	 */
> +	if (!ret || ret == -ENOSYS)
> +		arm_smmu_cb_write(smmu, idx, ARM_SMMU_CB_FSR, fsr);

Hmm, I don't grok this part. If the fault handler returned an error and
we don't clear the FSR, won't we just re-take the irq immediately? I think
it would be better to do this unconditionally, and print the "Unhandled
context fault" message for any non-zero value of ret.

Will