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 21:08:42 +0200 |
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On Sun, Jul 17 2022 at 00:22, Andrew Cooper wrote: >> -void load_percpu_segment(int cpu) >> +static noinstr void load_percpu_segment(int cpu) >> { >> #ifdef CONFIG_X86_32 >> loadsegment(fs, __KERNEL_PERCPU); >> #else >> __loadsegment_simple(gs, 0); >> - wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu)); >> + /* >> + * Because of the __loadsegment_simple(gs, 0) above, any GS-prefixed >> + * instruction will explode right about here. As such, we must not have >> + * any CALL-thunks using per-cpu data. >> + * >> + * Therefore, use native_wrmsrl() and have XenPV take the fault and >> + * emulate. >> + */ >> + native_wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu)); >> #endif > > Lovely :-/ > > But I still don't see how that works, because __loadsegment_simple() is > a memory clobber and cpu_kernelmode_gs_base() has a per-cpu lookup in > it.
No. It uses an array lookup :)
> That said, this only has a sole caller, and in context, it's bogus for > 64bit. Can't we fix all the problems by just doing this: > > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c > index 736262a76a12..6f393bc9d89d 100644 > --- a/arch/x86/kernel/cpu/common.c > +++ b/arch/x86/kernel/cpu/common.c > @@ -701,16 +701,6 @@ static const char *table_lookup_model(struct > cpuinfo_x86 *c) > __u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned > long)); > __u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long)); > > -void load_percpu_segment(int cpu) > -{ > -#ifdef CONFIG_X86_32 > - loadsegment(fs, __KERNEL_PERCPU); > -#else > - __loadsegment_simple(gs, 0); > - wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu)); > -#endif > -} > - > #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.
Thanks,
tglx
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