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Re: [PATCH 1/3 gnumach] apic: Use cpuid to read the apic id for speed
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From: |
Damien Zammit |
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Subject: |
Re: [PATCH 1/3 gnumach] apic: Use cpuid to read the apic id for speed |
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Date: |
Sat, 12 Aug 2023 00:56:10 +0000 |
Hi,
On 12/8/23 04:27, Luca wrote:
> How much faster is this? did you measure also on hw? I would use RDTSC
> to do it, I don't know if there are better ways.
So, ~8 microseconds was reduced to ~60 nanoseconds, so roughly 100 times faster.
I don't know how to do it with rdtsc.
> Interestingly, Linux uses either LSL or RDPID to obtain the same value,
> I wonder if they could be an easy alternative.
No idea about these.
> Also, could it be that there is a particular use of it that is much more
> frequent than the others, and that might benefit from further optimizations?
Perhaps, but it still makes sense not to use LAPIC mmio registers when a cpuid
instruction exists.
Damien