On Wed, Mar 22, 2017 at 12:36 AM, Ingo Molnar wrote: > > * Thomas Garnier wrote: > >> > static inline void setup_fixmap_gdt(int cpu) >> > { >> > __set_fixmap(get_cpu_gdt_ro_index(cpu), >> > - __pa(get_cpu_gdt_rw(cpu)), pg_fixmap_gdt_flags); >> > + slow_virt_to_phys(get_cpu_gdt_rw(cpu)), >> > + pg_fixmap_gdt_flags); >> > } >> > >> > /* Load the original GDT from the per-cpu structure */ >> > >> > This makes UP boot for me, but SMP (2 cpus) is still busted. >> >> This change fixed boot for me: >> >> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h >> index b65155cc3760..4e30707d9f9a 100644 >> --- a/arch/x86/include/asm/fixmap.h >> +++ b/arch/x86/include/asm/fixmap.h >> @@ -104,7 +104,9 @@ enum fixed_addresses { >> FIX_GDT_REMAP_BEGIN, >> FIX_GDT_REMAP_END = FIX_GDT_REMAP_BEGIN + NR_CPUS - 1, >> >> - __end_of_permanent_fixed_addresses, >> + __end_of_permanent_fixed_addresses = >> + (FIX_GDT_REMAP_END + PTRS_PER_PTE - 1) & >> + -PTRS_PER_PTE, >> >> Just ensure PKMAP_BASE & FIX_WP_TEST are on a different PMD. >> >> I don't think that the right fix but it might help understand the >> exact root cause. > > Could this be related to the permission bits in the PMD itself getting out of sync > with the PTEs? WP test marks a page writable/unwritable, and maybe we mess up the > restoration. If they are on separate PMDs then this is worked around because the > fixmap GDT is on a separate PMD. > I don't think so. I think it's a pair of bugs related to the way that percpu areas are virtually mapped. Bug 1: __pa is totally bogus on percpu pointers. Oddly, we have one older instance of exactly the same bug (on the same GDT address) in the kernel. I'll send a patch. Bug 2: Nothing syncs a freshly-set-up CPU's percpu area into initial_page_table. This makes access to the gdt fail in startup_32_smp. This looks like a longstanding bug, and I don't see what it has to do with Thomas' series. I'm still mulling over what to do about it. --Andy