Re: [PATCH 08/11] x86: entry_64.S: fold test_in_nmi macro into its only user

From: Steven Rostedt
Date: Wed Feb 11 2015 - 21:17:18 EST

On Wed, 11 Feb 2015 12:40:36 -0800
Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

> On Wed, Jan 14, 2015 at 1:48 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> > No code changes.
> Steven, is this okay with you?

Not as is. The reason I created the macro in the first place was to
give names to values and labels. That is:

- .macro test_in_nmi reg stack nmi_ret normal_ret
- cmpq %\reg, \stack
- ja \normal_ret
- subq $EXCEPTION_STKSZ, %\reg
- cmpq %\reg, \stack
- jb \normal_ret
- jmp \nmi_ret

Shows you that we are comparing the reg to a given stack. And it also
shows us where to jump as a result (normal_ret vs nmi_ret).

We lose that with:

+ cmpq %rdx, 4*8(%rsp)
+ ja first_nmi
+ subq $EXCEPTION_STKSZ, %rdx
+ cmpq %rdx, 4*8(%rsp)
+ jb first_nmi
+ jmp nested_nmi

I'm not against the change. I would like to see some descriptive
comments along with it. Like adding:

/* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
cmpq %rdx, 4*8(%rsp)
/* If the stack pointer is above the NMI stack, this is a normal NMI */
ja first_nmi
cmpq %rdx, 4*8(%rsp)
/* If it is below the NMI stack, it is a normal NMI */
jb first_nmi
/* Ah, it is within the NMI stack, treat it as nested */
jmp nested_nmi

-- Steve
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