From e4a84be6f05eab4778732d799f63b3cd15427885 Mon Sep 17 00:00:00 2001 From: Dave Hansen Date: Thu, 7 Jul 2016 17:19:14 -0700 Subject: x86/mm: Disallow running with 32-bit PTEs to work around erratum The Intel(R) Xeon Phi(TM) Processor x200 Family (codename: Knights Landing) has an erratum where a processor thread setting the Accessed or Dirty bits may not do so atomically against its checks for the Present bit. This may cause a thread (which is about to page fault) to set A and/or D, even though the Present bit had already been atomically cleared. These bits are truly "stray". In the case of the Dirty bit, the thread associated with the stray set was *not* allowed to write to the page. This means that we do not have to launder the bit(s); we can simply ignore them. If the PTE is used for storing a swap index or a NUMA migration index, the A bit could be misinterpreted as part of the swap type. The stray bits being set cause a software-cleared PTE to be interpreted as a swap entry. In some cases (like when the swap index ends up being for a non-existent swapfile), the kernel detects the stray value and WARN()s about it, but there is no guarantee that the kernel can always detect it. When we have 64-bit PTEs (64-bit mode or 32-bit PAE), we were able to move the swap PTE format around to avoid these troublesome bits. But, 32-bit non-PAE is tight on bits. So, disallow it from running on this hardware. I can't imagine anyone wanting to run 32-bit non-highmem kernels on this hardware, but disallowing them from running entirely is surely the safe thing to do. Signed-off-by: Dave Hansen Cc: Andrew Morton Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Dave Hansen Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Josh Poimboeuf Cc: Linus Torvalds Cc: Luis R. Rodriguez Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Toshi Kani Cc: dave.hansen@intel.com Cc: linux-mm@kvack.org Cc: mhocko@suse.com Link: http://lkml.kernel.org/r/20160708001914.D0B50110@viggo.jf.intel.com Signed-off-by: Ingo Molnar --- arch/x86/boot/cpu.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'arch/x86/boot/cpu.c') diff --git a/arch/x86/boot/cpu.c b/arch/x86/boot/cpu.c index 29207f69ae8c..26240dde081e 100644 --- a/arch/x86/boot/cpu.c +++ b/arch/x86/boot/cpu.c @@ -93,6 +93,8 @@ int validate_cpu(void) show_cap_strs(err_flags); putchar('\n'); return -1; + } else if (check_knl_erratum()) { + return -1; } else { return 0; } -- cgit v1.2.3-70-g09d2