On Thu, Oct 02, 2014 at 11:20:12PM +0800, Chen Yucong wrote:
> From: Chen Yucong <[email protected]>
> Subject: [PATCH] x86, MCE, AMD: move invariant code out from loop body
>
> "mce_threshold_vector = amd_threshold_interrupt;" is loop invariant code
> in mce_amd_feature_init(). So it should be moved out from loop body.
>
> Signed-off-by: Chen Yucong <[email protected]>
> ---
> arch/x86/kernel/cpu/mcheck/mce_amd.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/mcheck/mce_amd.c
> b/arch/x86/kernel/cpu/mcheck/mce_amd.c
> index 5d4999f..f727701 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce_amd.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce_amd.c
> @@ -253,9 +253,10 @@ void mce_amd_feature_init(struct cpuinfo_x86 *c)
> }
>
> mce_threshold_block_init(&b, offset);
> - mce_threshold_vector = amd_threshold_interrupt;
> }
> }
> +
> + mce_threshold_vector = amd_threshold_interrupt;
Looking at this more, it is theoretically possible that we break out
of the both loops without *any* thresholding registers detected and to
still assign a thresholding interrupt vector which would be clearly
wrong.
Thus I think something like below should be much safer (I tried it with
a label and goto already but it is uglier):
diff --git a/arch/x86/kernel/cpu/mcheck/mce_amd.c
b/arch/x86/kernel/cpu/mcheck/mce_amd.c
index 9ce64955559d..9af7bd74828b 100644
--- a/arch/x86/kernel/cpu/mcheck/mce_amd.c
+++ b/arch/x86/kernel/cpu/mcheck/mce_amd.c
@@ -253,7 +253,9 @@ void mce_amd_feature_init(struct cpuinfo_x86 *c)
}
mce_threshold_block_init(&b, offset);
- mce_threshold_vector = amd_threshold_interrupt;
+
+ if (mce_threshold_vector != amd_threshold_interrupt)
+ mce_threshold_vector = amd_threshold_interrupt;
}
}
}
Looking at the asm, we still go and fetch those addresses so not really
a win:
cmpq $amd_threshold_interrupt, mce_threshold_vector(%rip) #,
mce_threshold_vector
je .L235 #,
incl %r13d # block
movq $amd_threshold_interrupt, mce_threshold_vector(%rip) #,
mce_threshold_vector
cmpl $9, %r13d #, block
but this way the code is relatively clean. Unless you can come up with
a nicer, cleaner version to handle the breaking out in the success and
failure case...
Hmmm.
--
Regards/Gruss,
Boris.
Sent from a fat crate under my desk. Formatting is fine.
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