PMD update corruption (sync question)

[jcm@redhat.com (Jon Masters)]

On 03/03/2015 04:06 AM, Arnd Bergmann wrote:
> On Monday 02 March 2015 17:21:26 Jon Masters wrote:
>> On 03/02/2015 05:50 AM, Catalin Marinas wrote:
>>> On Mon, Mar 02, 2015 at 12:58:36AM -0500, Jon Masters wrote:
>>
>>>> Test kernels running with an explicit DSB in all PTE update cases now
>>>> running overnight. Just in case.
>>
>> ...and stay up after 19 hours. But that's just timing I'm sure.
>>
>>> It could be hiding some other problems.
>>
>> I checked my GDB macros and they were correct BUT my debugger went out
>> to lunch soon after that dump so I suspect it was just garbage 
>>
>> Instead, for my immediate issue, I have a much more likely suspect. For
>> anyone interested in the followup, you should know that hardware page
>> table walkers generally do respond well when you feed them Makefiles:
>>
>> 0x43e81c0000: 20230a23 656b614d 656c6966 726f6620  : #.# Makefile for
>> 0x43e81c0010: 65687420 462d4920 6563726f 69726420  :  the I-Force dri
>> 0x43e81c0020: 0a726576 20230a23 4a207942 6e61686f  : ver.#.# By Johan
>> 0x43e81c0030: 6544206e 7875656e 6f6a3c20 6e6e6168  : n Deneux <johann
>> 0x43e81c0040: 6e65642e 40787565 69616d67 6f632e6c  : .deneux at gmail.co
>> 0x43e81c0050: 230a3e6d 626f0a0a 28242d6a 464e4f43  : m>.#..obj-$(CONF
>> 0x43e81c0060: 4a5f4749 5453594f 5f4b4349 524f4649  : IG_JOYSTICK_IFOR
>> 0x43e81c0070: 09294543 69203d2b 63726f66 0a6f2e65  : CE).+= iforce.o.
>> 0x43e81c0080: 6f66690a 2d656372 3d3a2079 6f666920  : .iforce-y := ifo
>> 0x43e81c0090: 2d656372 6f2e6666 6f666920 2d656372  : rce-ff.o iforce-
>> 0x43e81c00a0: 6e69616d 69206f2e 63726f66 61702d65  : main.o iforce-pa
>> 0x43e81c00b0: 74656b63 0a6f2e73 726f6669 242d6563  : ckets.o.iforce-$
>> 0x43e81c00c0: 4e4f4328 5f474946 53594f4a 4b434954  : (CONFIG_JOYSTICK
>> 0x43e81c00d0: 4f46495f 5f454352 29323332 203d2b09  : _IFORCE_232).+=
>> 0x43e81c00e0: 726f6669 732d6563 6f697265 690a6f2e  : iforce-serio.o.i
>> 0x43e81c00f0: 63726f66 28242d65 464e4f43 4a5f4749  : force-$(CONFIG_J
>> 0x43e81c0100: 5453594f 5f4b4349 524f4649 555f4543  : OYSTICK_IFORCE_U
>> 0x43e81c0110: 09294253 69203d2b 63726f66 73752d65  : SB).+= iforce-us
>> 0x43e81c0120: 0a6f2e62 00000000 00000000 00000000  : b.o.............
>>
>> So that explains why things were falling over. It is likely indeed the
>> bad DMA I have been craving all along. And this time it was so gracious
>> as to give me the answer in plain ASCII  I suspect there will be a
>> patch for a certain AHCI driver in the not too distant future.
> 
> I hope this kind of problem becomes easier to debug once we have
> full iommu support working on arm64. When we had problems like this
> on PowerPC, using iommu=force to ensure DMA would only be done to
> pages that are currently mapped to the device was really helpful.

Oh, you can imagine that I put my best Dr. Evil hat on this week and
have my finger on the button already. In fact if I have my way future
SBSA compliant systems will be required to use an IOMMU with no way to
avoid having one. Whether that was before or after I was reduced to
walking kernel memory one word at a time and using pen and paper to
derive the above...We've a couple of years of "robustness investment"
ahead on ARM servers to ensure that we catch all of these things. And we
will catch all of them. And it will be utterly perfect.

Jon.