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


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC PATCH v2 1/2] utils: Add helper to read arm MIDR_EL1 register
Date: Thu, 18 Aug 2016 11:39:29 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0


On 18/08/2016 11:01, Vijay Kilari wrote:
> On Thu, Aug 18, 2016 at 2:20 PM, Paolo Bonzini <address@hidden> wrote:
>>
>>
>> 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.
> 
> Yes, but I could not think of better way to get rid of this check.

    bool need_aa64_prefetch = is_thunderx_pass2();
    for (...) {
         if (need_aa64_prefetch) {
             ...
         }
    }

The check on cpu_info_read is done just once.

Paolo

 However
> as Richard suggested (in another email), to drop this check and let prefetch
> be called for all the arm64 architectures. But I don't have any other
> arm64 platform
> to check the impact of it.
> 
>>
>> 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.
>>



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