RE: [PATCH v12 5/5] iommu/vt-d: improve ITE fault handling if target device isn't present

[Tian, Kevin]

> From: Ethan Zhao <haifeng.zhao@xxxxxxxxxxxxxxx>
> Sent: Monday, January 29, 2024 11:49 AM
> 
> Because surprise removal could happen anytime, e.g. user could request safe
> removal to EP(endpoint device) via sysfs and brings its link down to do
> surprise removal cocurrently. such aggressive cases would cause ATS
> invalidation request issued to non-existence target device, then deadly
> loop to retry that request after ITE fault triggered in interrupt context.
> this patch aims to optimize the ITE handling by checking the target device
> presence state to avoid retrying the timeout request blindly, thus avoid
> hard lockup or system hang.
> 
> Signed-off-by: Ethan Zhao <haifeng.zhao@xxxxxxxxxxxxxxx>
> ---
>  drivers/iommu/intel/dmar.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
> index 814134e9aa5a..2e214b43725c 100644
> --- a/drivers/iommu/intel/dmar.c
> +++ b/drivers/iommu/intel/dmar.c
> @@ -1272,6 +1272,7 @@ static int qi_check_fault(struct intel_iommu
> *iommu, int index, int wait_index,
>  {
>  	u32 fault;
>  	int head, tail;
> +	u64 iqe_err, ite_sid;
>  	struct q_inval *qi = iommu->qi;
>  	int shift = qi_shift(iommu);
> 
> @@ -1316,6 +1317,13 @@ static int qi_check_fault(struct intel_iommu
> *iommu, int index, int wait_index,
>  		tail = readl(iommu->reg + DMAR_IQT_REG);
>  		tail = ((tail >> shift) - 1 + QI_LENGTH) % QI_LENGTH;
> 
> +		/*
> +		 * SID field is valid only when the ITE field is Set in FSTS_REG
> +		 * see Intel VT-d spec r4.1, section 11.4.9.9
> +		 */
> +		iqe_err = dmar_readq(iommu->reg + DMAR_IQER_REG);
> +		ite_sid = DMAR_IQER_REG_ITESID(iqe_err);
> +
>  		writel(DMA_FSTS_ITE, iommu->reg + DMAR_FSTS_REG);
>  		pr_info("Invalidation Time-out Error (ITE) cleared\n");
> 
> @@ -1325,6 +1333,16 @@ static int qi_check_fault(struct intel_iommu
> *iommu, int index, int wait_index,
>  			head = (head - 2 + QI_LENGTH) % QI_LENGTH;
>  		} while (head != tail);
> 
> +		/*
> +		 * If got ITE, we need to check if the sid of ITE is the same as
> +		 * current ATS invalidation target device, if yes, don't try this
> +		 * request anymore if the target device isn't present.
> +		 * 0 value of ite_sid means old VT-d device, no ite_sid value.
> +		 */
> +		if (pdev && ite_sid && !pci_device_is_present(pdev) &&
> +			ite_sid == pci_dev_id(pci_physfn(pdev)))
> +			return -ETIMEDOUT;
> +

since the hardware already reports source id leading to timeout, can't we
just find the pci_dev according to reported ite_sid? this is a slow path (either
due to device in bad state or removed) hence it's not necessary to add more
intelligence to pass the pci_dev in, leading to only a partial fix can be backported.

It's also more future-proof, say if one day the driver allows batching invalidation
requests for multiple devices then no need to pass in a list of devices.

Then it's easier to backport a full fix.