Re: reliably set vtype.vill


Tim Newsome
 

Thanks. spike infers vill from the other bits when executing vsetvl (link), so the only way to set vill is to find an illegal combination from the other bits. The spec doesn't seem to explicitly say that vill is writable, only that it gets set when the other bits are invalid. Did I miss something? Is a clarification needed?

Tim

On Mon, Nov 1, 2021 at 10:32 AM Krste Asanovic <krste@...> wrote:
Put that value in a register and use the vsetvl instruction which takes a register argument for vtype.

Krste

On Nov 1, 2021, at 9:46 AM, Tim Newsome <tim@...> wrote:

I'm trying to set vill, and I can't think of a reliable way to do it.

This comes up when OpenOCD (a debugger) connects to a RV32 target where vtype contains (for instance) 0x80000000. The user asks to read the vector registers, and the debugger wants to use vtype so it can shift the register. When done, it wants to restore the original value so that there is no observable impact to the debugging.

Is there a way that I can write 0x80000000 to vtype?

Tim

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