Re: [RFC PATCH gnumach] percpu area using gs segment

[Luca]

Hi,

Il 26/08/23 08:48, Damien Zammit ha scritto:
> diff --git a/i386/i386/cpu_number.c b/i386/i386/cpu_number.c
> index ef19e11f..241015b5 100644
> --- a/i386/i386/cpu_number.c
> +++ b/i386/i386/cpu_number.c
> @@ -20,11 +20,17 @@
>   #include <i386/smp.h>
>   #include <i386/cpu.h>
>   #include <i386/mp_desc.h>
> +#include <i386/percpu.h>
>   #include <kern/printf.h>
>   
>   #if NCPUS > 1
> -int cpu_number(void)
> +int cpu_number_slow(void)
>   {
>         return cpu_id_lut[apic_get_current_cpu()];
>   }

This should also be defined for non-smp, otherwise compilation fails (I 
only tested x86_64). Also I would make compilaiton explicitely fail 
somewhere on x86_64+smp for now.

> diff --git a/i386/i386/percpu.c b/i386/i386/percpu.c
> new file mode 100644
> index 00000000..0bc8b234
> --- /dev/null
> +++ b/i386/i386/percpu.c
> @@ -0,0 +1,30 @@
> +/*
> + * Copyright (c) 2023 Free Software Foundation, Inc.
> + *
> + * This program is free software: you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation, either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +#include <i386/smp.h>
> +#include <i386/apic.h>
> +#include <i386/percpu.h>
> +
> +struct percpu percpu_array[NCPUS] __aligned(0x8000);

Why do you need this alignment?


> diff --git a/i386/i386/percpu.h b/i386/i386/percpu.h
> new file mode 100644
> index 00000000..b22d512c
> --- /dev/null
> +++ b/i386/i386/percpu.h
> @@ -0,0 +1,72 @@
> +/*
> + * Copyright (c) 2023 Free Software Foundation, Inc.
> + *
> + * This program is free software: you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation, either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#ifndef _PERCPU_H_
> +#define _PERCPU_H_
> +
> +#include <kern/ast.h>
> +#include <kern/processor.h>
> +#include <kern/thread.h>
> +#include <kern/timer.h>
> +#include <i386/mp_desc.h>
> +#include <i386/spl.h>
> +#include <intel/pmap.h>
> +#include <ipc/ipc_kmsg.h>
> +
> +#define percpu_assign(stm, val)     \
> +    asm("mov %0, %%gs:%1"           \
> +                 : : "r" (val), "m" (__builtin_offsetof(struct percpu, stm)));
> +
> +#define percpu_ptr(typ, stm)        \
> +MACRO_BEGIN                         \
> +    typ *ptr_ = (typ *)__builtin_offsetof(struct percpu, stm); \
> +                                    \
> +    asm("add %%gs:0, %0"            \
> +                 : "+r" (ptr_)      \
> +                 : );               \
> +                                    \
> +    ptr_;                           \
> +MACRO_END

If it could simplify this accessor, gcc (and also clang, if we ever need 
it) supports using a specific segment base for a variable:

https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Named-Address-Spaces.html#x86-Named-Address-Spaces

and

https://kernel.org/doc/html/next/x86/x86_64/fsgs.html#compiler-support-for-fs-gs-based-addressing


Luca