Re: [PATCH] target/arm: Make WFI a NOP for userspace emulators

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

On Fri, 30 Apr 2021 at 18:18, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 4/30/21 9:22 AM, Peter Maydell wrote:
> > The WFI insn is not system-mode only, though it doesn't usually make
> > a huge amount of sense for userspace code to execute it.  Currently
> > if you try it in qemu-arm then the helper function will raise an
> > EXCP_HLT exception, which is not covered by the switch in cpu_loop()
> > and results in an abort:
> >
> > qemu: unhandled CPU exception 0x10001 - aborting
> > R00=00000001 R01=408003e4 R02=408003ec R03=000102ec
> > R04=00010a28 R05=00010158 R06=00087460 R07=00010158
> > R08=00000000 R09=00000000 R10=00085b7c R11=408002a4
> > R12=408002b8 R13=408002a0 R14=0001057c R15=000102f8
> > PSR=60000010 -ZC- A usr32
> > qemu:handle_cpu_signal received signal outside vCPU context @ pc=0x7fcbfa4f0a12
> >
> > Make the WFI helper function return immediately in the usermode
> > emulator. This turns WFI into a NOP, which is OK because:
> >   * architecturally "WFI is a NOP" is a permitted implementation
> >   * aarch64 Linux kernels use the SCTLR_EL1.nTWI bit to trap
> >     userspace WFI and NOP it (though aarch32 kernels currently
> >     just let WFI do whatever it would do)
> >
> > We could in theory make the translate.c code special case user-mode
> > emulation and NOP the insn entirely rather than making the helper
> > do nothing, but because no real world code will be trying to
> > execute WFI we don't care about efficiency and the helper provides
> > a single place where we can make the change rather than having
> > to touch multiple places in translate.c and translate-a64.c.
> >
> > Fixes:https://bugs.launchpad.net/qemu/+bug/1926759
> > Signed-off-by: Peter Maydell<peter.maydell@linaro.org>
> > ---
> >   target/arm/op_helper.c | 12 ++++++++++++
> >   1 file changed, 12 insertions(+)
>
> You could also ifdef this out in translate, in tb_stop for  DISAS_WFI.

You'd need to do it in both translate.c and translate-a64.c if
you did it there, though.

thanks
-- PMM