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

[Daniel Henrique Barboza]

On 7/5/23 18:49, Conor Dooley wrote:
> 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.

You mean this link?

https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/riscv/cpus.yaml


> 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.


To make it required, as far as QEMU is concerned, we'll need to assume a
default value for the 'host' CPU type (e.g. sv57). In the future we can read the
satp host value directly when/if KVM provides satp_mode via get_one_reg().


Thanks,

Daniel

> 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.