Messages in this thread |  | | From | Thomas Gleixner <> | Subject | Re: [patch 02/38] x86/cpu: Use native_wrmsrl() in load_percpu_segment() | Date | Sun, 17 Jul 2022 22:08:16 +0200 |
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On Sun, Jul 17 2022 at 21:08, Thomas Gleixner wrote: > On Sun, Jul 17 2022 at 00:22, Andrew Cooper wrote: >> #ifdef CONFIG_X86_32 >> /* The 32-bit entry code needs to find cpu_entry_area. */ >> DEFINE_PER_CPU(struct cpu_entry_area *, cpu_entry_area); >> @@ -742,12 +732,15 @@ EXPORT_SYMBOL_GPL(load_fixmap_gdt); >> * Current gdt points %fs at the "master" per-cpu area: after this, >> * it's on the real one. >> */ >> -void switch_to_new_gdt(int cpu) >> +void __noinstr switch_to_new_gdt(int cpu) >> { >> /* Load the original GDT */ >> load_direct_gdt(cpu); >> + >> +#ifdef CONFIG_X86_32 >> /* Reload the per-cpu base */ >> - load_percpu_segment(cpu); >> + loadsegment(fs, __KERNEL_PERCPU); >> +#endif >> } >> >> static const struct cpu_dev *cpu_devs[X86_VENDOR_NUM] = {}; >> >> >> It's only 32bit where the percpu pointer is tied to the GDT. On 64bit, >> gsbase is good before this, and remains good after. >> >> With this change, >> >> # Make sure load_percpu_segment has no stackprotector >> CFLAGS_common.o := -fno-stack-protector >> >> comes up for re-evaluation too. > > Good point. Let me stare at it some more.
If it only would be that simple :)
loadsegment_simple() was a red herring. The gs segment is already zero.
So what explodes here is the early boot when switching from early per CPU to the real per CPU area.
start_kernel() ..... setup_per_cpu_areas(); smp_prepare_boot_cpu() switch_to_new_gdt() load_direct_gdt(cpu); load_percpu_segment(cpu) GS: 0 GS_BASE: 0xffffffff829d0000 (early PERCPU) wrmsrl() GS_BASE: 0xffff888237c00000 (real PERCPU)
So the explosion happens when accessing a per CPU variable after loading the GDT and before GS_BASE is fixed up.
That's the only case AFAICT where this matters. In all other invocations GS_BASE is already correct.
Let me fix this proper.
Thanks,
tglx
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