Re: masking of CSR bits/fields
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The current Priv spec effectively allows implementation flexibility (for better or worse) and it's quite reasonable to expect that some implementations answered this question one way, other implementations answered another way, and yet other implementations answered differently in different places (i.e. whatever is convenient of the design). Hence this, or any, mandating of a specific behavior will not be backward compatible with all existing and in-flight implementations - and instead will make a fraction (whether a majority or a minority) of implementations incompatible. This isn't something to be doing in Priv 1.12 as it is about to be ratified. But this would be good food for community discussion and input for a possible fast-track arch extension or maybe (?) as part of a "Priv 1.13".
And, if anything, there has been more clamoring over the past couple of years for possibly narrowing down similar sorts of behaviors around what is allowed with WARL fields (starting with the basics of when legalization is performed, and especially when legal values for a field are dependent on other arch state and can change). The point being that there are a number of areas (like these) where the architecture provides lots of flexibility and/or is under-specified (depending on one's viewpoint).
On Fri, Nov 12, 2021 at 7:41 PM John Hauser <jh.riscv@...> wrote:
Hi Privileged ISA group,