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

[Yicong Yang <yangyicong@hisilicon.com>]

Hi Robin,

On 2021/11/16 19:37, Yicong Yang wrote:
> On 2021/11/16 18:56, Robin Murphy wrote:
>> 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...
>>
> 
> our bios will reserve such a region and reported it through iort. the device will write to the region and in the driver we need to access the region
> to get the traced data. the region is reserved exclusively and will not be accessed by kernel or other devices.
> 
> is it ok to let bios configure the address to the device and from CPU side we just read it?
> 

Any suggestion?  Is this still an issue you concern if we move the configuration of the device address to BIOS and just read from the CPU side?

> 
> 
> .
>