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Re: [PATCH v2 2/3] target/riscv: Implement the stval/mtval illegal instr

From: Alistair Francis
Subject: Re: [PATCH v2 2/3] target/riscv: Implement the stval/mtval illegal instruction
Date: Wed, 29 Sep 2021 13:56:41 +1000

On Fri, Sep 24, 2021 at 10:57 PM Richard Henderson
<richard.henderson@linaro.org> wrote:
> On 9/24/21 2:48 AM, Alistair Francis wrote:
> >> But... more specific to this case.  Prior to this, was the exception 
> >> handler allowed to
> >> assume anything about the contents of stval?  Should the value have been 
> >> zero?  Would it
> >> be wrong to write to stval unconditionally?  How does the guest OS know 
> >> that it can rely
> >> on stval being set?
> >
> > As we didn't support writing the illegal instruction stval should be
> > zero before this patch.
> Ok, that didn't quite answer the question...
> If *wasn't* zero before this patch: we didn't write anything at all, and so 
> keep whatever
> previous value the previous exception wrote.
> Is that a bug that needs fixing?  Because you're still not writing anything 
> to stval if

Yeah, that sounds like a bug then.

> >> I simply wonder whether it's worthwhile to add the feature and feature 
> >> test.
> >
> > Do you just mean have it enabled all the time?
> Yes, if without this feature the value of stval was undefined.

Ok, I'll have another look at this. Thanks for pointing this out.


> r~

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