linux-kernel - Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack

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Message-ID: <CALCETrVaaGywcHoZ=QCYCLKrkiH2sN9T7dX+=m-iPZKp3pSXWg@mail.gmail.com>
Date:   Tue, 16 Jan 2018 14:37:08 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack

On Tue, Jan 16, 2018 at 12:30 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 16 Jan 2018, Joerg Roedel wrote:
>> @@ -89,13 +89,9 @@ static inline void refresh_sysenter_cs(struct thread_struct *thread)
>>  /* This is used when switching tasks or entering/exiting vm86 mode. */
>>  static inline void update_sp0(struct task_struct *task)
>>  {
>> -     /* On x86_64, sp0 always points to the entry trampoline stack, which is constant: */
>> -#ifdef CONFIG_X86_32
>> -     load_sp0(task->thread.sp0);
>> -#else
>> +     /* sp0 always points to the entry trampoline stack, which is constant: */
>>       if (static_cpu_has(X86_FEATURE_XENPV))
>>               load_sp0(task_top_of_stack(task));
>> -#endif
>>  }
>>
>>  #endif /* _ASM_X86_SWITCH_TO_H */
>> diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
>> index 654229bac2fc..7270dd834f4b 100644
>> --- a/arch/x86/kernel/asm-offsets_32.c
>> +++ b/arch/x86/kernel/asm-offsets_32.c
>> @@ -47,9 +47,11 @@ void foo(void)
>>       BLANK();
>>
>>       /* Offset from the sysenter stack to tss.sp0 */
>> -     DEFINE(TSS_sysenter_stack, offsetof(struct cpu_entry_area, tss.x86_tss.sp0) -
>> +     DEFINE(TSS_sysenter_stack, offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
>>              offsetofend(struct cpu_entry_area, entry_stack_page.stack));

I was going to say that this is just too magical.  The convention is
that STRUCT_member refers to "member" of "STRUCT".  Here you're
encoding a more complicated calculation.  How about putting just the
needed offsets in asm_offsets and putting the actual calculation in
the asm code or a header.

>>
>> +     OFFSET(TSS_sp1, tss_struct, x86_tss.sp1);

This belongs in asm_offsets.c.  Just move the asm_offsets_64.c version
there and call it a day.