Re: [PATCH] Semihost SYS_READC implementation (v4)

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 14 Nov 2019 at 22:27, Keith Packard <keithp@keithp.com> wrote:
>
> Richard Henderson <richard.henderson@linaro.org> writes:
>
> > For semi-hosting, it seems even better if the semi-hosting syscall instruction
> > is not "real", because you're explicitly requesting services from "unreal"
> > hardware.  It should be specified to generate a SIGILL type of exception
> > anywhere semi-hosting is not enabled.
>
> In the QEMU case, yes, it's virtual hardware. However, the other common case
> for semihosting is when doing hardware bringup using JTAG or other
> remote debugging link -- having an instruction which safely traps to the
> debugger is required to make semihosting usable there. Reading through
> the history of the current RISC-V semihosting mechanism, there were many
> designs considered and rejected because they wouldn't work in the JTAG
> environment.
>
> > With that in mind, it may be simpler to handle all of this not in the
> > translator, but in the function that delivers the ebreak exception.  At that
> > point one can arrange to read memory without raising additional
> > exceptions.
>
> I'll go explore and see if I can figure any of this out.
>
> I'd still like to get the non-RISC-V SYS_READC patch landed someday :-)

It's on my queue to review if nobody else gets to it first, but since
we're in freeze right now it won't be landing til after the release
happens (expected mid-December).

thanks
-- PMM