<arnd>
Having RISC-V cores of their own fits nicely in that strategy
<sorear>
not sure about the cost effectiveness
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<TwoNotes>
WHy would I be getting an 'illegal instruction' exception on an 'mret' instrcution while in M-mode? (QEMU sifive_u54)
<sorear>
not actually in m-mode? bad setting in mstatus?
<sorear>
look at helper_mret in op_helper.c
<TwoNotes>
I know I am in M mode because this is early in boot phase. Maybe I set MPP wrong. I am trying to drop to S mode
<sorear>
hmm, qemu throws an illegal instruction if you don't have any PMP rules configured, pretty sure that's nonconformant but you need to do that anyway
<sorear>
(PMP behavior in S-mode is default deny; if qemu didn't special case this, mret with the PMP unconfigured would result in an immediate instruction access fault taken from S-mode)
<TwoNotes>
WHat is PMP? Is that a QEMU thing?
<sorear>
physical memory protection, look it up in the privileged spec
<TwoNotes>
Ok, I know that term
<TwoNotes>
Looking at op_helper, it even requires the mepc value to be 8-byte aligned
<TwoNotes>
Ooops no, I mean 4-byte
<jrtc27>
there's some dodginess in QEMU's implementation of xEPC legalisation
<jrtc27>
but if you do normal things it works
<jrtc27>
it doesn't need to be 4-byte aligned if you have compressed instructions
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<TwoNotes>
I see where it checks for PMP rules, but only if MPP is not M. I guess to make sure PMP has been set up *before* changing out of M mode.
<TwoNotes>
Since I have just set MPP to 'S' on purpose, that would trigger the fault
* TwoNotes
goes off to read the PMP chapter again
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