Re: [PATCH v2 04/22] target/riscv: Improve fidelity of guest external interrupts

[Anup Patel]

On Thu, Sep 9, 2021 at 12:14 PM Alistair Francis <alistair23@gmail.com> wrote:
>
> On Thu, Sep 2, 2021 at 9:26 PM Anup Patel <anup.patel@wdc.com> wrote:
> >
> > The guest external interrupts for external interrupt controller are
> > not delivered to the guest running under hypervisor on time. This
> > results in a guest having sluggish response to serial console input
> > and other I/O events. To improve timely delivery of guest external
> > interrupts, we check and inject interrupt upon every sret instruction.
> >
> > Signed-off-by: Anup Patel <anup.patel@wdc.com>
> > ---
> >  target/riscv/op_helper.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
> > index ee7c24efe7..4c995c239e 100644
> > --- a/target/riscv/op_helper.c
> > +++ b/target/riscv/op_helper.c
> > @@ -129,6 +129,15 @@ target_ulong helper_sret(CPURISCVState *env, 
> > target_ulong cpu_pc_deb)
> >
> >      riscv_cpu_set_mode(env, prev_priv);
> >
> > +    /*
> > +     * QEMU does not promptly deliver guest external interrupts
> > +     * to a guest running on a hypervisor which in-turn is running
> > +     * on QEMU. We make dummy call to riscv_cpu_update_mip() upon
> > +     * every sret instruction so that QEMU pickup guest external
> > +     * interrupts sooner.
> > +     */
> > +     riscv_cpu_update_mip(env_archcpu(env), 0, 0);
>
> This doesn't seem right. I don't understand why we need this?
>
> riscv_cpu_update_mip() is called when an interrupt is delivered to the
> CPU, if we are missing interrupts then that is a bug somewhere else.

I have finally figured out the cause of guest external interrupts being
missed by Guest/VM.

The riscv_cpu_set_irq() which handles guest external interrupt lines
of each CPU is called asynchronously. This function in-turn calls
riscv_cpu_update_mip() but the CPU might be in host mode (V=0)
or in Guest/VM mode (V=1). If the CPU is in host mode (V=0) when
the riscv_cpu_set_irq() is called, then the CPU interrupt requested by
riscv_cpu_update_mip() has no effect because the CPU can't take
the interrupt until it enters Guest/VM mode.

This patch does the right thing by doing a dummy call to
riscv_cpu_update_mip() upon SRET instruction so that if the CPU
had missed a guest interrupt previously then the CPU can take it now.

Regards,
Anup