Re: ftrace use of pci_resource_to_user()

[karol herbst]

2016-05-06 12:16 GMT+02:00 Pekka Paalanen <ppaalanen@xxxxxxxxx>:
> On Wed, 4 May 2016 14:17:13 -0500
> Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
>> 138295373ccf ("ftrace: mmiotrace update, #2") added this use of
>> pci_resource_to_user():
>>
>>   +static int mmio_print_pcidev(struct trace_seq *s, const struct pci_dev *dev)
>>   +{
>>   ...
>>   +       /*
>>   +        * XXX: is pci_resource_to_user() appropriate, since we are
>>   +        * supposed to interpret the __ioremap() phys_addr argument based on
>>   +        * these printed values?
>>   +        */
>>   +       for (i = 0; i < 7; i++) {
>>   +               pci_resource_to_user(dev, i, &dev->resource[i], &start, &end);
>>   +               ret += trace_seq_printf(s, " %llx",
>>   +                       (unsigned long long)(start |
>>   +                       (dev->resource[i].flags & PCI_REGION_FLAG_MASK)));
>>   +       }
>>
>> I think it was a mistake to use pci_resource_to_user() here because it
>> adds unnecessary arch dependencies in whatever consumes this output.
>>
>> On most arches, pci_resource_to_user() is a no-op and the result is
>> normal resource addresses, i.e., CPU physical addresses that match
>> things in /proc/iomem and /sys/devices/pci.../resource.
>>
>> On microblaze, mips, powerpc, and sparc, the result of
>> pci_resource_to_user() is something else, usually a PCI bus address (a
>> raw BAR value).  These values are only useful for using mmap on
>> files like /proc/bus/pci/....
>>
>> I don't know what, if anything, consumes this output.  If things parse
>> it, we shouldn't break them.  But those things likely would need
>> special cases for microblaze, mips, powerpc, and sparc.
>>
>> If it's only for human consumption, I think we should consider
>> removing the use of pci_resource_to_user() and printing
>> dev->resource[i].start instead.
>
> Hi,
>
> the code in question prints the "PCIDEV" lines in the mmiotrace output.
> IIRC, it was initially meant to replicate the contents
> of /proc/bus/pci/devices. I do not know it any tools rely on it, I
> suppose they might, for mapping MAPs to device and BAR.
>
> I am adding to CC some people that actually work with mmiotrace for
> Nouveau. It is used for seeing what the NVIDIA proprieratry driver
> does, so if there is no NVIDIA driver for the arch, they probably don't
> care. I am not sure if other driver projects use it too, IIRC I heard
> something about some wireless drivers in the past.
>
>
> Thanks,
> pq

Hi,

thanks for adding us here.

The code in question with which we parse the MMiotraces is demmio:
https://github.com/envytools/envytools/blob/master/rnn/demmio.c

I never really looked into, though, so it could be partly wrong what I say here.

In line 261 to 282 is the PCIDEV parsing section.
And this is used to get the relative address from the start of the BAR
regions, so that demmio can look addresses up in rnndb (database of
the mmio registers of Nvidia GPUs)
and prints out which value is written/read at which time and what that
value means for the hardware.

And because that tools is kind of important for us to properly RE what
the nvidia does with the hardware, we really want to be carefull
changing anything here.

Thanks,

Karol