linux-kernel - Re: [patch 04/14] x86/exceptions: Make IST index zero based

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Date:   Tue, 2 Apr 2019 09:49:00 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [patch 04/14] x86/exceptions: Make IST index zero based

On Sun, Mar 31, 2019 at 11:40:24PM +0200, Thomas Gleixner wrote:
> The defines for the exception stack (IST) array in the TSS are using the
> SDM convention IST1 - IST7. That causes all sorts of code to subtract 1 for
> array indices related to IST. That's confusing at best and does not provide
> any value.
> 
> Make the indices zero based and fixup the usage sites. The only code which
> needs to adjust the 0 based index is the interrupt descriptor setup which
> needs to add 1 now.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  Documentation/x86/kernel-stacks      |    8 ++++----
>  arch/x86/entry/entry_64.S            |    4 ++--
>  arch/x86/include/asm/page_64_types.h |   13 ++++++++-----
>  arch/x86/kernel/cpu/common.c         |    4 ++--
>  arch/x86/kernel/dumpstack_64.c       |   14 +++++++-------
>  arch/x86/kernel/idt.c                |   15 +++++++++------
>  6 files changed, 32 insertions(+), 26 deletions(-)
> 
> --- a/Documentation/x86/kernel-stacks
> +++ b/Documentation/x86/kernel-stacks
> @@ -59,7 +59,7 @@ If that assumption is ever broken then t
>  
>  The currently assigned IST stacks are :-
>  
> -* DOUBLEFAULT_STACK.  EXCEPTION_STKSZ (PAGE_SIZE).
> +* DOUBLEFAULT_IST.  EXCEPTION_STKSZ (PAGE_SIZE).
>  
>    Used for interrupt 8 - Double Fault Exception (#DF).
>  
> @@ -68,7 +68,7 @@ The currently assigned IST stacks are :-
>    Using a separate stack allows the kernel to recover from it well enough
>    in many cases to still output an oops.
>  
> -* NMI_STACK.  EXCEPTION_STKSZ (PAGE_SIZE).
> +* NMI_IST.  EXCEPTION_STKSZ (PAGE_SIZE).
>  
>    Used for non-maskable interrupts (NMI).
>  
> @@ -76,7 +76,7 @@ The currently assigned IST stacks are :-
>    middle of switching stacks.  Using IST for NMI events avoids making
>    assumptions about the previous state of the kernel stack.
>  
> -* DEBUG_STACK.  DEBUG_STKSZ
> +* DEBUG_IST.  DEBUG_STKSZ

What about <name>_STACK_IST, or is that too verbose?  It'd be nice to have
the extra mental cue that IST is referring the 64 exception stacks.

>  
>    Used for hardware debug interrupts (interrupt 1) and for software
>    debug interrupts (INT3).
> @@ -86,7 +86,7 @@ The currently assigned IST stacks are :-
>    avoids making assumptions about the previous state of the kernel
>    stack.
>  
> -* MCE_STACK.  EXCEPTION_STKSZ (PAGE_SIZE).
> +* MCE_IST.  EXCEPTION_STKSZ (PAGE_SIZE).
>  
>    Used for interrupt 18 - Machine Check Exception (#MC).
>