Re: [Xen-devel] [PATCH 5/6] xen/arm: smccc: Add wrapper to automatically select the calling convention

[Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>]

On 28.08.18 17:43, Julien Grall wrote:
[...]

> > > I have looked at cpus_have_const_cap() and haven't found good way to 
> > > optimize it with the current infrastructure in Xen. Feel free to 
> > > suggest improvement.
> > Another thing: maybe it is worth to branch to 1.0 code and leave 1.1 
> > in a straight path of execution? This will save you one more 
> > instruction for SMCCC 1.1 call.
> I am not sure to understand your suggestion here. Could you expand?
+#define arm_smccc_smc(...)                                      \
+    do {                                                        \
+        if ( cpus_have_const_cap(ARM_SMCCC_1_1) )              \
+            arm_smccc_1_1_smc(__VA_ARGS__);                     \
+        else                                                    \
+            arm_smccc_1_0_smc(__VA_ARGS__);                     \
+    } while ( 0 )


> However, why SMCCC 1.1 should be in the straight path of execution?
It is easier to read - no negation in if(). Also, I think, branch 
predictor would be happy. At least, if the following is true:
" If the branch information is not contained in the BTAC, static branch 
prediction is used, whereby we assume the branch will be taken if the 
branch is a conditional backwards branch or not taken if the branch is a 
conditional forwards branch." [1]

[1] 
http://linuxkernelhacker.blogspot.com/2014/07/branch-prediction-logic-in-arm.html
--
Volodymyr Babchuk

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