Re: [PATCH] igb_uio: fix mmap failure

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 7/1/2016 1:47 PM, Thomas Monjalon wrote:
> Thank you Ferruh for taking care of igb_uio.
> 
> 2016-07-01 12:35, Ferruh Yigit:
>> With kernels enabled CONFIG_IO_STRICT_DEVMEM option mmap the iomem area
>> to userspace fails:
> 
> Maybe some words are missing.
> Please check punctuation of the whole commit message to make it easier
> to understand.
I will re-word.

> 
>> EAL: pci_map_resource():
>>          cannot mmap(39, 0x7f1c51800000, 0x100000, 0x0):
>>          Invalid argument (0xffffffffffffffff)
>>
>> As a workaround igb_uio can stop reserving PCI memory resources, from
>> kernel point of view io-memory region looks like idle and mmap works
>> again.
>>
>> With this update device io-memory range is not protected against any
>> other kernel driver claim ownership on those resources, which shouldn't
>> be a problem for dpdk usage module.
> 
> Why it should not be a problem?
request_mem_region() is a way for driver informing the rest of the
kernel that memory region is used.
And with CONFIG_IO_STRICT_DEVMEM=y, userspace also prevented to touch
that ares.
But for igb_uio, we explicitly want userspace to access that memory range.

> Please could you give an example of what could happen?
Technically device lost a protection of its memory region against any
other driver, but I am not sure if this is real threat in practical life.
Also this is same in uio_pci_generic, it doesn't reserve the memory.

> 
> This patch fixes a problem with recent kernels (not mentioned above)
> which offer the uio_pci_generic alternative.
Will give kernel version information.

> That's why I think we should fix it only if there is absolutely no
> regression for older kernels.
> 
Totally agreed, that is why I expressed my concern, let this patch hang
around a little.