QEMU 4.1.0 with SMP die option

Software & Operating Systems VFIO
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I just saw QEMU 4.1.0 now has an option to define the CPU dies ( https://wiki.qemu.org/ChangeLog/4.1 )
If I understand this correctly, this will let us define the die topoligy to the guest - so the guest can make better decitions about where to place threads (If the OS / Application support this)

Has anyone had time to play with this option?

My current system needs a replacement soon, and I as thinking about giving AMD a try again…
But, in my case - I am using minimal OS, and run all my guests virtual (both Linux, and sometimes WIndows) with GPU passthrough.
Currently I only assign 1 core (2 threads) to my “hypervisor OS”, and the rest goes to the guests.
I will assume on an AMD/Zen platform, it would be smarter to assign 1 core for each CCX to the underlaying OS… ?

1 Like
2

Do you mean CPU pinning? I didn’t realise that is so new

3

I don’t see what you are talking about on the change log

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In the x86 section of the changelog.

  • CPU die topology can now be configured using the -smp ...,dies=... option
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But all AMD-Zen are X86_64?

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Okay, I get the difference now, does it make any measurable difference to just cpu pinning?

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And is it easier than choosing pinned cores from Lstopo?

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It would seem Windows how has understanding of CCX… https://borncity.com/win/2019/06/30/windows-10-v1903-optimization-for-zen-based-amd-cpus/

Has anyone with a GPU Passthrough Ryzen system had a chance test this, with the newer “die” options to qemu ?
And if yes, can anyone confirm if it is possible to split the CCX uneven ? (like, having 3+4 configuration) ?

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Haven’t tried it still; I just use the whole cpu, and not having issues.

Can you try it and let us know?

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I do not have a Ryzen system yet… That is kinda why I am asking (As I was thinking of getting a 3700x system).
I just wanted to make sure, that everything will run good for VFIO as-well.

If you have a Ryzen 7 setup, could you comment in the CCX vs die option then please?

11

Has anyone had a chance to play with the die functions of qemu?