linux-kernel - Re: [PATCH 04/16] x86/fixmap: Generalize the GDT fixmap mechanism

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Message-ID: <CALCETrXXJaqeCtzEarESTtmk19gf0bxfQ3DsWZJ_UrBg=ZUeqg@mail.gmail.com>
Date:   Mon, 20 Nov 2017 17:21:34 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 04/16] x86/fixmap: Generalize the GDT fixmap mechanism

On Mon, Nov 20, 2017 at 2:01 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Mon, 20 Nov 2017, Andy Lutomirski wrote:
>
> Just a few nits.
>
>>  /* Provide the fixmap address of the remapped GDT */
>>  static inline struct desc_struct *get_cpu_gdt_ro(int cpu)
>>  {
>> -     unsigned int idx = get_cpu_gdt_ro_index(cpu);
>> -     return (struct desc_struct *)__fix_to_virt(idx);
>> +     return (struct desc_struct *)&get_cpu_entry_area(cpu)->gdt;
>>  }
>>
>>  /* Provide the current read-only GDT */
>> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
>> index b0c505fe9a95..5ff8e251772a 100644
>> --- a/arch/x86/include/asm/fixmap.h
>> +++ b/arch/x86/include/asm/fixmap.h
>> @@ -44,6 +44,17 @@ extern unsigned long __FIXADDR_TOP;
>>                        PAGE_SIZE)
>>  #endif
>>
>> +/*
>> + * cpu_entry_area is a percpu region in the fixmap that contains things
>> + * need by the CPU and early entry/exit code.  Real types aren't used here
>
> s/need/needed/

Yeah, fixed

>
>> + * to avoid circular header dependencies.
>
> :(

Hmm.  I could probably fix this, but it involves (at least) moving a
struct definition and adding several new includes, and I'm not sure
it'll actually converge to something  working.

>
>> + */
>> +struct cpu_entry_area
>> +{
>> +     char gdt[PAGE_SIZE];
>> +};
>> +
>> +#define CPU_ENTRY_AREA_PAGES (sizeof(struct cpu_entry_area) / PAGE_SIZE)
>
>> +static inline unsigned int __get_cpu_entry_area_page_index(int cpu, int page)
>> +{
>> +     BUILD_BUG_ON(sizeof(struct cpu_entry_area) % PAGE_SIZE != 0);
>> +
>> +     return FIX_CPU_ENTRY_AREA_BOTTOM - cpu*CPU_ENTRY_AREA_PAGES - page;
>> +}
>> +
>> +#define __get_cpu_entry_area_offset_index(cpu, offset) ({            \
>> +     BUILD_BUG_ON(offset % PAGE_SIZE != 0);                          \
>> +     __get_cpu_entry_area_page_index(cpu, offset / PAGE_SIZE);       \
>> +     })
>> +
>> +#define get_cpu_entry_area_index(cpu, field)                         \
>> +     __get_cpu_entry_area_offset_index((cpu), offsetof(struct cpu_entry_area, field))
>
> Any reason why those need to be macros?

The former is a macro because I doubt that BUILD_BUG_ON is valid in
that context in a function.  The latter is a macro because 'field' is
a name, not a value.

>
>> +static inline struct cpu_entry_area *get_cpu_entry_area(int cpu)
>> +{
>> +     return (struct cpu_entry_area *)
>> +             __fix_to_virt(__get_cpu_entry_area_page_index(cpu, 0));
>> +}
>> +
>
> Thanks,
>
>         tglx
>