Re: [Qemu-devel] [RFC PATCH v2 1/2] utils: Add helper to read arm MIDR_EL1 register

[Paolo Bonzini]

On 18/08/2016 09:56, Vijay Kilari wrote:
> The get_aarch_cpu_id() has check " if (unlikely(!cpu_info_read)) ".
> If we call get_aarch_cpu_id() from is_thunderx_pass2_cpu() which is
> called from inside the loop, we will be adding one additional check.

On the other hand, you are making an assumption that the caller of
is_thunderx_pass2_cpu() calls get_aarch64_cpu_id() first, and not
documenting it anywhere.

And given that you shouldn't call _any_ function from inside such a hot
loop, your solution is inferior on both counts.

Paolo

> What I observed is having extra check inside the loop is adding 100 to
> 200ms overhead
> on live migration time. So I added this variable extra is_thunderx_cpu
> static variable
> to make it simple single check.