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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Volodymyr,

On 28/08/18 16:10, Volodymyr Babchuk wrote:
> 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(). 
I can do that. I will also probably add an unlikely in 
cpus_have_const_cap(...).
> 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]
The Arm Arm does not provide any details on the branch predictor 
implementation. An implementer is free to do whatever he wants and could 
design a branch predicator doing the opposite as this statement.
You also can't assume how the compiler will compile the code, it may end 
up to generate the else branch first because it is predicted to be taken 
more often. This is why GCC provide __builtin_expect (commonly used as 
unlikely/likely) to influence the compiler choice for branch prediction.
Cheers,

--
Julien Grall

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