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Re: riscv64 system emulation maximum core count with virt machine

From: Mark Wyse
Subject: Re: riscv64 system emulation maximum core count with virt machine
Date: Tue, 5 Oct 2021 09:47:39 -0700

Thanks Rahul,

After some digging I came to the same conclusions. I'll dig deeper to see if there are any impacts in raising max CPU count for RISC-V Virt machine, but I definitely agree that it should be set higher.

FWIW, I was able to run a 32-core kernel after adjusting code and kernel appropriately and don't recall any warning or error messages popping up.


On Wed, Sep 29, 2021 at 10:16 AM Rahul Pathak <rpathakmailbox@gmail.com> wrote:
If you want to boot with more than 8 harts then you also need to increase the CONFIG_NR_CPUS in the kernel. Along with change in QEMU - VIRT_CPUS_MAX in  "include/hw/riscv/virt.h"

But, the limit of 8 is low and should be increased. Don't know the reason why its 8.

On Sep 22 2021, at 10:19 pm, Mark Wyse <wysem@cs.washington.edu> wrote:
Hello all,

Apologies if I missed the answer in the docs, but I am wondering why the virt machine is limited to only 8 cores when running RISC-V 64 system emulation? I am interested in booting a 16, 24, or even 32 core SMP Linux kernel with OpenSBI as the bootloader on the generic virt machine before adding a custom machine definition. I was able to successfully boot Linux kernel 5.9.0 with OpenSBI v0.9 and 8 cores/harts with an ext4 filesystem provided by Buildroot.

I poked around the docs and source code a bit, and found mention of the core count limit in the docs (https://qemu.readthedocs.io/en/latest/system/riscv/virt.html), but no reason as to why this is the limit. It also appears that providing a custom device tree blob and setting QEMU's -smp option to match might overcome this limitation, but it would be better if more than 8 cores was supported directly.


Mark Wyse
pronouns: he/him/his
PhD Student
Paul G. Allen School of Computer Science & Engineering
University of Washington

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