On 7/4/19 6:12 PM, Jon Masters wrote: > I think we have identified the root cause of the 32-bit builder issue. > Many thanks to Paul and Peter for assistance in debugging. Here's my > write-up, and we'll work with the vendor on a suitable mitigation to > workaround any errata: > > https://medium.com/@jonmasters_84473/debugging-a-32-bit-fedora-arm-builder-issue-73295d7d673d The hardware vendor have reproduced what I believe to be an errata. Meanwhile, I've made a test kernel that forces CONFIG_HIGHPTE to off: https://koji.fedoraproject.org/koji/taskinfo?taskID=36328838 With this kernel, you still get LPAE but leaf level PTEs are not allocated from high memory any longer. This is because I believe the errata to be caused by stage 1 page table walks in the guest trapping to stage 2 (hypervisor) for e.g. Access bit updates on the host. When those occur, I believe there is a truncation of the guest IPA (guest memory) address to 32-bits, but only for page table entry walks. Normal translation faults I think are unaffected by this (TBC). Normally, we don't allocate PGDs (high level page table pieces) from high memory (we allocate those from kernel memory caches) but we DO allocate PTEs specifically from what might be high memory. Except when we force CONFIG_HIGHPTE to off. The patch I'm using is attached. It's currently being tested. If it works, I'm curious for input on temporarily carrying this in Fedora. In theory it means an LPAE system could starve for PTEs if it has many many processes running, but in practice I'm willing to bet LPAE is mostly used by Fedora for the 32-bit builders and that few people would actually complain if we did this. Jon. -- Computer Architect | Sent with my Fedora powered laptop _______________________________________________ kernel mailing list -- kernel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to kernel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/kernel@xxxxxxxxxxxxxxxxxxxxxxx
