Re: [patch 02/38] x86/cpu: Use native_wrmsrl() in load_percpu_segment()

Juergen Gross
2022-07-18T05:11:36Z

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On 17.07.22 23:54, Thomas Gleixner wrote:
> On Sun, Jul 17 2022 at 22:13, Thomas Gleixner wrote:
>> On Sun, Jul 17 2022 at 22:08, Thomas Gleixner wrote:
>>> On Sun, Jul 17 2022 at 21:08, Thomas Gleixner wrote:
>>> 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()
>>
>> Bah. switch_to_new_gdt() is already invoked from setup_per_cpu_areas()
>> and then again in smp_prepare_boot_cpu() and once more in cpu_init(),
>>
>> What a mess.
>
> So the below builds and boots at least on 64bit. I'll stare at it some
> more tomorrow. I have no idea whether native_load_gdt() works with
> XEN_PV. It should, but what do I know.

No, shouldn't work. But ...

>
> Thanks,
>
> tglx
> ---
> --- a/arch/x86/include/asm/desc.h
> +++ b/arch/x86/include/asm/desc.h
> @@ -205,7 +205,7 @@ static inline void native_set_ldt(const
> }
> }
>
> -static inline void native_load_gdt(const struct desc_ptr *dtr)
> +static __always_inline void native_load_gdt(const struct desc_ptr *dtr)
> {
> asm volatile("lgdt %0"::"m" (*dtr));
> }
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -670,10 +670,9 @@ extern int sysenter_setup(void);
> /* Defined in head.S */
> extern struct desc_ptr early_gdt_descr;
>
> -extern void switch_to_new_gdt(int);
> +extern void switch_to_real_gdt(int);
> extern void load_direct_gdt(int);
> extern void load_fixmap_gdt(int);
> -extern void load_percpu_segment(int);
> extern void cpu_init(void);
> extern void cpu_init_secondary(void);
> extern void cpu_init_exception_handling(void);
> --- a/arch/x86/kernel/cpu/Makefile
> +++ b/arch/x86/kernel/cpu/Makefile
> @@ -7,20 +7,24 @@
> ifdef CONFIG_FUNCTION_TRACER
> CFLAGS_REMOVE_common.o = -pg
> CFLAGS_REMOVE_perf_event.o = -pg
> +CFLAGS_REMOVE_switch_gdt.o = -pg
> endif
>
> # If these files are instrumented, boot hangs during the first second.
> KCOV_INSTRUMENT_common.o := n
> KCOV_INSTRUMENT_perf_event.o := n
> +KCOV_INSTRUMENT_switch_gdt.o := n
>
> # As above, instrumenting secondary CPU boot code causes boot hangs.
> KCSAN_SANITIZE_common.o := n
> +KCSAN_SANITIZE_switch_gdt.o := n
>
> -# Make sure load_percpu_segment has no stackprotector
> -CFLAGS_common.o := -fno-stack-protector
> +# Make sure that switching the GDT and the per CPU segment
> +# does not have stack protector enabled.
> +CFLAGS_switch_gdt.o := -fno-stack-protector
>
> obj-y := cacheinfo.o scattered.o topology.o
> -obj-y += common.o
> +obj-y += common.o switch_gdt.o
> obj-y += rdrand.o
> obj-y += match.o
> obj-y += bugs.o
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -701,16 +701,6 @@ static const char *table_lookup_model(st
> __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);
> @@ -738,18 +728,6 @@ void load_fixmap_gdt(int cpu)
> }
> 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)
> -{
> - /* Load the original GDT */
> - load_direct_gdt(cpu);
> - /* Reload the per-cpu base */
> - load_percpu_segment(cpu);
> -}
> -
> static const struct cpu_dev *cpu_devs[X86_VENDOR_NUM] = {};
>
> static void get_model_name(struct cpuinfo_x86 *c)
> @@ -2228,12 +2206,6 @@ void cpu_init(void)
> boot_cpu_has(X86_FEATURE_TSC) || boot_cpu_has(X86_FEATURE_DE))
> cr4_clear_bits(X86_CR4_VME|X86_CR4_PVI|X86_CR4_TSD|X86_CR4_DE);
>
> - /*
> - * Initialize the per-CPU GDT with the boot GDT,
> - * and set up the GDT descriptor:
> - */
> - switch_to_new_gdt(cpu);
> -
> if (IS_ENABLED(CONFIG_X86_64)) {
> loadsegment(fs, 0);
> memset(cur->thread.tls_array, 0, GDT_ENTRY_TLS_ENTRIES * 8);
> --- /dev/null
> +++ b/arch/x86/kernel/cpu/switch_gdt.c
> @@ -0,0 +1,31 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#include <asm/processor.h>
> +#include <asm/segment.h>
> +#include <asm/desc.h>
> +
> +/*
> + * Invoked during early boot to switch from early GDT and early per CPU
> + * (%fs on 32bit, GS_BASE on 64bit) to the real GDT and the runtime per CPU
> + * area.
> + *
> + * This has to be done atomic because after switching from early GDT to
> + * the real one any per cpu variable access is going to fault because
> + * %fs resp. GS_BASE is not yet pointing to the real per CPU data.
> + *
> + * As a consequence this uses the native variants of load_gdt() and
> + * wrmsrl(). So XEN_PV has to take the fault and emulate.
> + */
> +void __init switch_to_real_gdt(int cpu)
> +{
> + struct desc_ptr gdt_descr;
> +
> + gdt_descr.address = (long)get_cpu_gdt_rw(cpu);
> + gdt_descr.size = GDT_SIZE - 1;
> + native_load_gdt(&gdt_descr);
> +
> +#ifdef CONFIG_X86_32
> + loadsegment(fs, __KERNEL_PERCPU);
> +#else
> + native_wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
> +#endif
> +}
> --- a/arch/x86/kernel/setup_percpu.c
> +++ b/arch/x86/kernel/setup_percpu.c
> @@ -211,7 +211,7 @@ void __init setup_per_cpu_areas(void)
> * area. Reload any changed state for the boot CPU.
> */
> if (!cpu)
> - switch_to_new_gdt(cpu);
> + switch_to_real_gdt(cpu);
> }
>
> /* indicate the early static arrays will soon be gone */
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1457,7 +1457,11 @@ void arch_thaw_secondary_cpus_end(void)
> void __init native_smp_prepare_boot_cpu(void)
> {
> int me = smp_processor_id();
> - switch_to_new_gdt(me);
> +
> + /* SMP invokes this from setup_per_cpu_areas() */
> + if (!IS_ENABLED(CONFIG_SMP))
> + switch_to_real_gdt(me);
> +
> /* already set me in cpu_online_mask in boot_cpu_init() */
> cpumask_set_cpu(me, cpu_callout_mask);
> cpu_set_state_online(me);
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1164,7 +1164,7 @@ static void __init xen_setup_gdt(int cpu
> pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry_boot;
> pv_ops.cpu.load_gdt = xen_load_gdt_boot;
>
> - switch_to_new_gdt(cpu);
> + switch_to_real_gdt(cpu);

... can't you use the paravirt variant of load_gdt in switch_to_real_gdt() ?

>
> pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry;
> pv_ops.cpu.load_gdt = xen_load_gdt;
>

Juergen

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