Re: Troubles running Xen on Raspberry Pi 4 with 5.6.1 DomU

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

+ Boris, Jürgen

See the crash Roman is seeing booting dom0 on the Raspberry Pi. It is
related to the recent xen dma_ops changes. Possibly the same thing
reported by Peng here:

https://marc.info/?l=linux-kernel&m=158805976230485&w=2

An in-depth analysis below.


On Mon, 4 May 2020, Roman Shaposhnik wrote:
> > > [    2.534292] Unable to handle kernel paging request at virtual
> > > address 000000000026c340
> > > [    2.542373] Mem abort info:
> > > [    2.545257]   ESR = 0x96000004
> > > [    2.548421]   EC = 0x25: DABT (current EL), IL = 32 bits
> > > [    2.553877]   SET = 0, FnV = 0
> > > [    2.557023]   EA = 0, S1PTW = 0
> > > [    2.560297] Data abort info:
> > > [    2.563258]   ISV = 0, ISS = 0x00000004
> > > [    2.567208]   CM = 0, WnR = 0
> > > [    2.570294] [000000000026c340] user address but active_mm is swapper
> > > [    2.576783] Internal error: Oops: 96000004 [#1] SMP
> > > [    2.581784] Modules linked in:
> > > [    2.584950] CPU: 3 PID: 135 Comm: kworker/3:1 Not tainted 
> > > 5.6.1-default #9
> > > [    2.591970] Hardware name: Raspberry Pi 4 Model B (DT)
> > > [    2.597256] Workqueue: events deferred_probe_work_func
> > > [    2.602509] pstate: 60000005 (nZCv daif -PAN -UAO)
> > > [    2.607431] pc : xen_swiotlb_free_coherent+0x198/0x1d8
> > > [    2.612696] lr : dma_free_attrs+0x98/0xd0
> > > [    2.616827] sp : ffff800011db3970
> > > [    2.620242] x29: ffff800011db3970 x28: 00000000000f7b00
> > > [    2.625695] x27: 0000000000001000 x26: ffff000037b68410
> > > [    2.631129] x25: 0000000000000001 x24: 00000000f7b00000
> > > [    2.636583] x23: 0000000000000000 x22: 0000000000000000
> > > [    2.642017] x21: ffff800011b0d000 x20: ffff80001179b548
> > > [    2.647461] x19: ffff000037b68410 x18: 0000000000000010
> > > [    2.652905] x17: ffff000035d66a00 x16: 00000000deadbeef
> > > [    2.658348] x15: ffffffffffffffff x14: ffff80001179b548
> > > [    2.663792] x13: ffff800091db37b7 x12: ffff800011db37bf
> > > [    2.669236] x11: ffff8000117c7000 x10: ffff800011db3740
> > > [    2.674680] x9 : 00000000ffffffd0 x8 : ffff80001071e980
> > > [    2.680124] x7 : 0000000000000132 x6 : ffff80001197a6ab
> > > [    2.685568] x5 : 0000000000000000 x4 : 0000000000000000
> > > [    2.691012] x3 : 00000000f7b00000 x2 : fffffdffffe00000
> > > [    2.696465] x1 : 000000000026c340 x0 : 000002000046c340
> > > [    2.701899] Call trace:
> > > [    2.704452]  xen_swiotlb_free_coherent+0x198/0x1d8
> > > [    2.709367]  dma_free_attrs+0x98/0xd0
> > > [    2.713143]  rpi_firmware_property_list+0x1e4/0x240
> > > [    2.718146]  rpi_firmware_property+0x6c/0xb0
> > > [    2.722535]  rpi_firmware_probe+0xf0/0x1e0
> > > [    2.726760]  platform_drv_probe+0x50/0xa0
> > > [    2.730879]  really_probe+0xd8/0x438
> > > [    2.734567]  driver_probe_device+0xdc/0x130
> > > [    2.738870]  __device_attach_driver+0x88/0x108
> > > [    2.743434]  bus_for_each_drv+0x78/0xc8
> > > [    2.747386]  __device_attach+0xd4/0x158
> > > [    2.751337]  device_initial_probe+0x10/0x18
> > > [    2.755649]  bus_probe_device+0x90/0x98
> > > [    2.759590]  deferred_probe_work_func+0x88/0xd8
> > > [    2.764244]  process_one_work+0x1f0/0x3c0
> > > [    2.768369]  worker_thread+0x138/0x570
> > > [    2.772234]  kthread+0x118/0x120
> > > [    2.775571]  ret_from_fork+0x10/0x18
> > > [    2.779263] Code: d34cfc00 f2dfbfe2 d37ae400 8b020001 (f8626800)
> > > [    2.785492] ---[ end trace 4c435212e349f45f ]---
> > > [    2.793340] usb 1-1: New USB device found, idVendor=2109,
> > > idProduct=3431, bcdDevice= 4.20
> > > [    2.801038] usb 1-1: New USB device strings: Mfr=0, Product=1, 
> > > SerialNumber=0
> > > [    2.808297] usb 1-1: Product: USB2.0 Hub
> > > [    2.813710] hub 1-1:1.0: USB hub found
> > > [    2.817117] hub 1-1:1.0: 4 ports detected
> > >
> > > This is bailing out right here:
> > >      
> > > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/firmware/raspberrypi.c?h=v5.6.1#n125
> > >
> > > FYIW (since I modified the source to actually print what was returned
> > > right before it bails) we get:
> > >    buf[1] == 0x800000004
> > >    buf[2] == 0x00000001
> > >
> > > Status 0x800000004 is of course suspicious since it is not even listed 
> > > here:
> > >     
> > > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/soc/bcm2835/raspberrypi-firmware.h#n14
> > >
> > > So it appears that this DMA request path is somehow busted and it
> > > would be really nice to figure out why.
> >
> > You have actually discovered a genuine bug in the recent xen dma rework
> > in Linux. Congrats :-)
> 
> Nice! ;-)
> 
> > I am doing some guesswork here, but from what I read in the thread and
> > the information in this email I think this patch might fix the issue.
> > If it doesn't fix the issue please add a few printks in
> > drivers/xen/swiotlb-xen.c:xen_swiotlb_free_coherent and please let me
> > know where exactly it crashes.
> >
> >
> > diff --git a/include/xen/arm/page-coherent.h 
> > b/include/xen/arm/page-coherent.h
> > index b9cc11e887ed..ff4677ed9788 100644
> > --- a/include/xen/arm/page-coherent.h
> > +++ b/include/xen/arm/page-coherent.h
> > @@ -8,12 +8,17 @@
> >  static inline void *xen_alloc_coherent_pages(struct device *hwdev, size_t 
> > size,
> >                 dma_addr_t *dma_handle, gfp_t flags, unsigned long attrs)
> >  {
> > +       void *cpu_addr;
> > +       if (dma_alloc_from_dev_coherent(hwdev, size, dma_handle, &cpu_addr))
> > +               return cpu_addr;
> >         return dma_direct_alloc(hwdev, size, dma_handle, flags, attrs);
> >  }
> >
> >  static inline void xen_free_coherent_pages(struct device *hwdev, size_t 
> > size,
> >                 void *cpu_addr, dma_addr_t dma_handle, unsigned long attrs)
> >  {
> > +       if (dma_release_from_dev_coherent(hwdev, get_order(size), cpu_addr))
> > +               return;
> >         dma_direct_free(hwdev, size, cpu_addr, dma_handle, attrs);
> >  }
> 
> Applied the patch, but it didn't help and after printk's it turns out
> it surprisingly crashes right inside this (rather convoluted if you
> ask me) if statement:
>     
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/xen/swiotlb-xen.c?h=v5.6.1#n349
> 
> So it makes sense that the patch didn't help -- we never hit that
> xen_free_coherent_pages.
 
The crash happens here:

        if (!WARN_ON((dev_addr + size - 1 > dma_mask) ||
                     range_straddles_page_boundary(phys, size)) &&
            TestClearPageXenRemapped(virt_to_page(vaddr)))
                xen_destroy_contiguous_region(phys, order);

I don't know exactly what is causing the crash. Is it the WARN_ON somehow?
Is it TestClearPageXenRemapped? Neither should cause a crash in theory.


But I do know that there are problems with that if statement on ARM. It
can trigger for one of the following conditions:

1) dev_addr + size - 1 > dma_mask
2) range_straddles_page_boundary(phys, size)


The first condition might happen after bef4d2037d214 because
dma_direct_alloc might not respect the device dma_mask. It is actually a
bug and I would like to keep the WARN_ON for that. The patch I sent
yesterday (https://marc.info/?l=xen-devel&m=158865080224504) should
solve that issue. But Roman is telling us that the crash still persists.

The second condition is completely normal and not an error on ARM
because dom0 is 1:1 mapped. It is not an issue if the address range is
straddling a page boundary. We certainly shouldn't WARN (or crash).

So, I suggest something similar to Peng's patch, appended.

Roman, does it solve your problem?


diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index b6d27762c6f8..994ca3a4b653 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -346,9 +346,8 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t 
size, void *vaddr,
        /* Convert the size to actually allocated. */
        size = 1UL << (order + XEN_PAGE_SHIFT);
 
-       if (!WARN_ON((dev_addr + size - 1 > dma_mask) ||
-                    range_straddles_page_boundary(phys, size)) &&
-           TestClearPageXenRemapped(virt_to_page(vaddr)))
+       WARN_ON(dev_addr + size - 1 > dma_mask);
+       if (TestClearPageXenRemapped(virt_to_page(vaddr)))
                xen_destroy_contiguous_region(phys, order);
 
        xen_free_coherent_pages(hwdev, size, vaddr, (dma_addr_t)phys, attrs);