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SubjectRe: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch
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Hi,

Thanks for taking a look at this.

On 5/1/20 2:05 AM, Greg KH wrote:
> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
>> On arm64, and possibly other architectures, requesting
>> IO coherent memory may return Normal-NC if the underlying
>> hardware isn't coherent. If these pages are then
>> remapped into userspace as Normal, that defeats the
>> purpose of getting Normal-NC, as well as resulting in
>> mappings with differing cache attributes.
>
> What is "Normal-NC"?

A non-cacheable attribute on arm64 pages. I think Mark R & Marc Z
elaborated while I was asleep (thanks!).
.


>
>> In particular this happens with libusb, when it attempts
>> to create zero-copy buffers as is used by rtl-sdr, and
>
> What is "rtl-sdr"

Its the realtek software defined radio (SDR), a really inexpensive TV
dongle that was discovered could be used as a general purpose SDR a
decade or so ago. In particular, this project
https://github.com/osmocom/rtl-sdr/
which is packaged by fedora/etc.

>
>> maybe other applications. The result is usually
>> application death.
>
> So is this a new problem? Old problem? Old problem only showing up on
> future devices? On current devices? I need a hint here as to know if
> this is a bugfix or just work to make future devices work properly.

This has been a problem on arm devices without IO coherent USB
apparently for years. The rtl-sdr project itself has a disable zero-copy
mode that people have been using on rpi/etc specific builds. Fedora
OTOH, is building it with the same flags on x86 & arm64 which means that
people report problems. This happened a few days ago (on a pinebook),
and I duplicated it on an NXP platform just running the `rtl_test`
artifact with a nooelec from my junk box. Guessing that it was a page
mismatch I went looking for that, rather than disabling the zero copy
since punishing arm machine that have IO coherent USB adapters for the
sins of these low end devices isn't ideal. I found this, and this patch
allows the rtl_test app to run without issues on my NXP/solidrun.

Plus, given that its actually a kernel/libusb problem its likely there
are other applications having similar problems.

>
>>
>> If dma_mmap_attr() is used instead of remap_pfn_range,
>> the page cache/etc attributes can be matched between the
>> kernel and userspace.
>>
>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>> ---
>> drivers/usb/core/devio.c | 5 ++---
>> 1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
>> index 6833c918abce..1e7458dd6e5d 100644
>> --- a/drivers/usb/core/devio.c
>> +++ b/drivers/usb/core/devio.c
>> @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>> {
>> struct usb_memory *usbm = NULL;
>> struct usb_dev_state *ps = file->private_data;
>> + struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
>> size_t size = vma->vm_end - vma->vm_start;
>> void *mem;
>> unsigned long flags;
>> @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>> usbm->vma_use_count = 1;
>> INIT_LIST_HEAD(&usbm->memlist);
>>
>> - if (remap_pfn_range(vma, vma->vm_start,
>> - virt_to_phys(usbm->mem) >> PAGE_SHIFT,
>> - size, vma->vm_page_prot) < 0) {
>> + if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
>
> Given that this code has not changed since 2016, how has no one noticed
> this issue before?
They have there are a lot of reports of sdr failures, but the general
use case is rare?

>
> And have you tested this change out on other systems (i.e. x86) to
> ensure that this still works properly?

Yes and no, I did some basic libusb tests on an x86 machine, but its a
bit tricky at the moment for me to get the rtl plugged into a x86 test
machine. (its a work in progress).


>
> And why isn't this call used more by drivers if this is a real issue?
The particulars of asking for iocoherent memory and then mapping it to
userspace is rarer than just asking for kmalloc()/remap() and then
performing the dma ops?

Then there are all the softer issues around arm64 testing/availability
and vendors carrying "fixes" for particular issues (like rtl-sdr
disabling zero copy).

> And will this cause issues with how the userspace mapping is handled as
> now we rely on userspace to do things differently? Or am I reading the
> dma_mmap_attrs() documentation wrong?
I don't think userspace is doing anything differently here, and AFAIK,
on systems with IO coherent adapters this ends up with the same page
mapping as just doing the remap_pfn_rage() with the same attributes as
before. I've looked at dma_map_attrs() a bit, but i'm also trusting it
does what it says on the tin.


Thanks again for looking at this.

>
> thanks,
>
> greg k-h
>

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