Re: Re: [PATCH v3 01/21] hw/riscv: Use misa_mxl instead of misa_mxl_max

[Andrew Jones]

On Wed, Jan 24, 2024 at 12:08:33PM +0900, Akihiko Odaki wrote:
> On 2024/01/23 17:20, Andrew Jones wrote:
> > On Mon, Jan 22, 2024 at 02:55:50PM +0000, Alex Bennée wrote:
> > > From: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > 
> > > The effective MXL value matters when booting.
> > 
> > I'd prefer this commit message get some elaboration. riscv_is_32bit()
> > is used in a variety of contexts, some where it should be reporting
> > the max misa.mxl. However, when used for booting an S-mode kernel it
> > should indeed report the effective mxl. I think we're fine with the
> > change, though, because at init and on reset the effective mxl is set
> > to the max mxl, so, in those contexts, where riscv_is_32bit() should
> > be reporting the max, it does.
> > 
> > > 
> > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > Message-Id: <20240103173349.398526-23-alex.bennee@linaro.org>
> > > Message-Id: <20231213-riscv-v7-1-a760156a337f@daynix.com>
> > > Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> > > ---
> > >   hw/riscv/boot.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/riscv/boot.c b/hw/riscv/boot.c
> > > index 0ffca05189f..bc67c0bd189 100644
> > > --- a/hw/riscv/boot.c
> > > +++ b/hw/riscv/boot.c
> > > @@ -36,7 +36,7 @@
> > >   bool riscv_is_32bit(RISCVHartArrayState *harts)
> > >   {
> > > -    return harts->harts[0].env.misa_mxl_max == MXL_RV32;
> > > +    return harts->harts[0].env.misa_mxl == MXL_RV32;
> > >   }
> > 
> > Assuming everyone agrees with what I've written above, then maybe we
> > should write something similar in a comment above this function.
> > 
> > Thanks,
> > drew
> 
> The corresponding commit in my series has a more elaborated message:
> https://patchew.org/QEMU/20240115-riscv-v9-0-ff171e1aedc8@daynix.com/20240115-riscv-v9-1-ff171e1aedc8@daynix.com/

I've pulled the message from that link and quoted it below

> A later commit requires one extra step to retrieve misa_mxl_max. As
> misa_mxl is semantically more correct and does not need such a extra
> step, refer to misa_mxl instead. Below is the explanation why misa_mxl
> is more semantically correct to refer to than misa_mxl_max in this case.
> 
> Currently misa_mxl always equals to misa_mxl_max so it does not matter

That's true, but I think that's due to a bug in write_misa(), which
shouldn't be masking val with the extension mask until mxl has been
extracted.

> which of misa_mxl or misa_mxl_max to refer to. However, it is possible
> to have different values for misa_mxl and misa_mxl_max if QEMU gains a
> new feature to load a RV32 kernel on a RV64 system, for example. For
> such a behavior, the real system will need the firmware to switch MXL to
> RV32, and if QEMU implements the same behavior, mxl will represent the
> MXL that corresponds to the kernel being loaded. Therefore, it is more
> appropriate to refer to mxl instead of misa_mxl_max when
> misa_mxl != misa_mxl_max.

Right, but that doesn't say anything more than the original one line,
"The effective MXL value matters when booting."

What I'm looking for is a code comment explaining how riscv_is_32bit()
is always safe to use. Something like

 /*
  * Checking the effective mxl is always correct, because the effective
  * mxl will be equal to the max mxl at initialization and also on reset,
  * which are the times when it should check the maximum mxl. Later, if
  * firmware writes misa with a smaller mxl, then that mxl should be
  * used in checks.
  */

Thanks,
drew