Re: [PATCH v2 2/6] hwtracing: Add trace function support for HiSilicon PCIe Tune and Trace device

[Robin Murphy <robin.murphy@arm.com>]

On 2021-11-16 09:06, Yicong Yang via iommu wrote:
[...]
> +/*
> + * Get RMR address if provided by the firmware.
> + * Return 0 if the IOMMU doesn't present or the policy of the
> + * IOMMU domain is passthrough or we get a usable RMR region.
> + * Otherwise a negative value is returned.
> + */
> +static int hisi_ptt_get_rmr(struct hisi_ptt *hisi_ptt)
> +{
> +	struct pci_dev *pdev = hisi_ptt->pdev;
> +	struct iommu_domain *iommu_domain;
> +	struct iommu_resv_region *region;
> +	LIST_HEAD(list);
> +
> +	/*
> +	 * Use direct DMA if IOMMU does not present or the policy of the
> +	 * IOMMU domain is passthrough.
> +	 */
> +	iommu_domain = iommu_get_domain_for_dev(&pdev->dev);
> +	if (!iommu_domain || iommu_domain->type == IOMMU_DOMAIN_IDENTITY)
> +		return 0;
> +
> +	iommu_get_resv_regions(&pdev->dev, &list);
> +	list_for_each_entry(region, &list, list)
> +		if (region->type == IOMMU_RESV_DIRECT &&
> +		    region->length >= HISI_PTT_TRACE_BUFFER_SIZE) {
> +			hisi_ptt->trace_ctrl.has_rmr = true;
> +			hisi_ptt->trace_ctrl.rmr_addr = region->start;
> +			hisi_ptt->trace_ctrl.rmr_length = region->length;
> +			break;
> +		}
> +
> +	iommu_put_resv_regions(&pdev->dev, &list);
> +	return hisi_ptt->trace_ctrl.has_rmr ? 0 : -ENOMEM;
> +}

No.

The whole point of RMRs is for devices that are already configured to 
access the given address range in a manner beyond the kernel's control. 
If you can do this, it proves that you should not have an RMR in the 
first place.

The notion of a kernel driver explicitly configuring its device to DMA 
into any random RMR that looks big enough is so egregiously wrong that 
I'm almost lost for words...

Robin.