On Wed, Mar 22, 2017 at 9:33 AM, Andy Lutomirski wrote: > 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. Why do you think padding the fixmap also fix the problem? That's the thing I don't get. With the padding the PA is now correct and the memcmp check also succeed. That's odd. > > --Andy -- Thomas