Re: [PATCH v8 02/20] hw/riscv/virt.c: skip 'mmu-type' FDT if satp mode not set

[Conor Dooley]

On Wed, Jul 05, 2023 at 06:39:37PM -0300, Daniel Henrique Barboza wrote:
> The absence of a satp mode in riscv_host_cpu_init() is causing the
> following error:
> 
> $ ./qemu/build/qemu-system-riscv64  -machine virt,accel=kvm \
>     -m 2G -smp 1  -nographic -snapshot \
>     -kernel ./guest_imgs/Image \
>     -initrd ./guest_imgs/rootfs_kvm_riscv64.img \
>     -append "earlycon=sbi root=/dev/ram rw" \
>     -cpu host
> **
> ERROR:../target/riscv/cpu.c:320:satp_mode_str: code should not be
> reached
> Bail out! ERROR:../target/riscv/cpu.c:320:satp_mode_str: code should
> not be reached
> Aborted
> 
> The error is triggered from create_fdt_socket_cpus() in hw/riscv/virt.c.
> It's trying to get satp_mode_str for a NULL cpu->cfg.satp_mode.map.
> 
> For this KVM cpu we would need to inherit the satp supported modes
> from the RISC-V host. At this moment this is not possible because the
> KVM driver does not support it. And even when it does we can't just let
> this broken for every other older kernel.
> 
> Since mmu-type is not a required node, according to [1], skip the
> 'mmu-type' FDT node if there's no satp_mode set. We'll revisit this
> logic when we can get satp information from KVM.
> 
> [1] 
> https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/cpu.yaml

I don't think this is the correct link to reference as backup, as the
generic binding sets out no requirements. I think you would want to link
to the RISC-V specific cpus binding.

That said, things like FreeBSD and U-Boot appear to require mmu-type
https://lore.kernel.org/all/20230705-fondue-bagginess-66c25f1a4135@spud/
so I am wondering if we should in fact make the mmu-type a required
property in the RISC-V specific binding.

Since nommu is covered by an mmu type of "riscv,none", I am kinda
struggling to think of a case where it should be left out (while
describing real hardware at least).

Cheers,
Conor.

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