On 28/05/2025 11:48, David Hildenbrand wrote: > On 28.05.25 12:44, David Hildenbrand wrote: >> On 28.05.25 12:34, Ryan Roberts wrote: >>> Hi David, >>> >>> >>> On 09/05/2025 16:30, David Hildenbrand wrote: >>>> Let's test some basic functionality using /dev/mem. These tests will >>>> implicitly cover some PAT (Page Attribute Handling) handling on x86. >>>> >>>> These tests will only run when /dev/mem access to the first two pages >>>> in physical address space is possible and allowed; otherwise, the tests >>>> are skipped. >>> >>> We are seeing really horrible RAS errors with this test when run on arm64 >>> tx2 >>> machine. Based solely on reviewing the code, I think the problem is that tx2 >>> doesn't have anything at phys address 0, so test_read_access() is trying to >>> put >>> trasactions out to a bad address on the bus. >>> >>> tx2 /proc/iomem: >>> >>> $ sudo cat /proc/iomem >>> 30000000-37ffffff : PCI ECAM >>> 38000000-3fffffff : PCI ECAM >>> 40000000-5fffffff : PCI Bus 0000:00 >>> ... >>> >>> Whereas my x86 box has some reserved memory: >>> >>> $ sudo cat /proc/iomem >>> 00000000-00000fff : Reserved >>> 00001000-0003dfff : System RAM >>> ... >>> >> >> A quick fix would be to make this test specific to x86 (the only one I >> tested on). We should always have the lower two pages IIRC (BIOS stuff etc).
I'm not sure how far along this patch is? I'm guessing mm-stable? Perhaps you can do the quick fix, then I'd be happy to make this more robust for arm64 later? >> >>> I think perhaps the only safe way to handle this is to parse /proc/iomem >>> for a >>> region of "System RAM" that is at least 2 pages then use that for your read >>> tests. This would also solve the hypothetical issue of reading something >>> that >>> has read size effects. >> >> That sounds also plausible yes. I somehow remembered that mmap() would >> fail if "there is nothing". > > Ah, my memory comes back, we perform checks only with CONFIG_STRICT_DEVMEM. Ahh makes sense. I guess our config doesn't include this. I just checked the RAS error and it is for PA 0. So I'm confident that what I describe above is definitely what is happening.
