Re: [PATCH v4 17/33] target/nios2: Prevent writes to read-only or reserved control fields

[Richard Henderson]

On 3/8/22 00:57, Peter Maydell wrote:
> On Tue, 8 Mar 2022 at 07:20, Richard Henderson
> <richard.henderson@linaro.org> wrote:
> > Create an array of masks which detail the writable and readonly
> > bits for each control register.  Apply them when writing to
> > control registers.
> >
> > Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>
> What's the justification for this extra machinery? Does
> existing guest code rely on writes to r/o bits being ignored ?

During review of a previous edition of this patch set, I asked myself: why isn't Amir 
changing the shadow register set on WRCTL to STATUS, as the CRS field could change.

The answer is that the architecture disallows this, by making the CRS field read-only from 
software.  CRS can only be modified by interrupt entry and return.

Then I looked further and found more read-only fields, and lots of fields that are 
reserved unless the appropriate cpu options are enabled.  Again thining of CRS more so to 
PRS when shadow register sets are *not* enabled -- we don't want software to put us into 
some weird state.

I probably should have put all that in the patch description.  :-)


r~