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 07:00:52PM -0300, Daniel Henrique Barboza wrote:
> 
> 
> 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

Yeah, that's the correct file. Should probably have linked it, sorry
about that. And in case it was not clear, not suggesting that this would
require a resend, since the reasoning is correct.

> > 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().

I dunno if assuming is the right thing to do, since it could be actively
wrong. Leaving it out, as you are doing here, is, IMO, nicer to those
guests. Once there's an API for it, I think it could then be added and
then the additional guests would be supported.

Thanks,
Conor.

signature.asc
Description: PGP signature