Re: [PATCH v6 1/5] hw/riscv: Use misa_mxl instead of misa_mxl_max

[Akihiko Odaki]

On 2023/11/23 12:04, Alistair Francis wrote:
> On Mon, Oct 30, 2023 at 3:50 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> > The effective MXL value matters when booting.
>
> This doesn't sound right. Surely the max is what matters here
>
> Also, this was specifically changed to misa_mxl_max in db23e5d981a
> "target/riscv: Replace riscv_cpu_is_32bit with riscv_cpu_mxl".
>
> This needs a much better description of why this change should be made
>
> Alistair

The kernel will be executed with the current MXL rather than the initial 
MXL value so the current MXL should be used here.

For example, if you are going to emulate a system that has a RV64 CPU 
and a firmware that sets the MXL to RV32, then mxl_max should be 
MXL_RV64 and mxl should be MXL_RV32, and the kernel should be assumed as 
a RV32 binary. Loading a 64-bit kernel will not work in such a case.

You can find a similar example in x86_64: x86_64 systems typically 
starts in 16-bit mode, and the firmware switches to 64-bit mode. When 
emulating those systems, QEMU switches to 64-bit mode and loads a 64-bit 
kernel.

Regards,
Akihiko Odaki