Re: [RFC PATCH V1 01/12] hvm/ioreq: Make x86's IOREQ feature common

[Oleksandr <olekstysh@gmail.com>]

On 06.08.20 03:37, Stefano Stabellini wrote:

Hi Stefano

Trying to simulate IO_RETRY handling mechanism (according to model 
below) I continuously get IO_RETRY from try_fwd_ioserv() ...

> OK, thanks for the details. My interpretation seems to be correct.
>
> In which case, it looks like xen/arch/arm/io.c:try_fwd_ioserv should
> return IO_RETRY. Then, xen/arch/arm/traps.c:do_trap_stage2_abort_guest
> also needs to handle try_handle_mmio returning IO_RETRY the first
> around, and IO_HANDLED when after QEMU does its job.
>
> What should do_trap_stage2_abort_guest do on IO_RETRY? Simply return
> early and let the scheduler do its job? Something like:
>
>              enum io_state state = try_handle_mmio(regs, hsr, gpa);
>
>              switch ( state )
>              {
>              case IO_ABORT:
>                  goto inject_abt;
>              case IO_HANDLED:
>                  advance_pc(regs, hsr);
>                  return;
>              case IO_RETRY:
>                  /* finish later */
>                  return;
>              case IO_UNHANDLED:
>                  /* IO unhandled, try another way to handle it. */
>                  break;
>              default:
>                  ASSERT_UNREACHABLE();
>              }
>
> Then, xen/arch/arm/ioreq.c:handle_mmio() gets called by
> handle_hvm_io_completion() after QEMU completes the emulation. Today,
> handle_mmio just sets the user register with the read value.
>
> But it would be better if it called again the original function
> do_trap_stage2_abort_guest to actually retry the original operation.
> This time do_trap_stage2_abort_guest calls try_handle_mmio() and gets
> IO_HANDLED instead of IO_RETRY,
I may miss some important point, but I failed to see why try_handle_mmio 
(try_fwd_ioserv) will return IO_HANDLED instead of IO_RETRY at this stage.
Or current try_fwd_ioserv() logic needs rework?


> thus, it will advance_pc (the program
> counter) completing the handling of this instruction.
>
> The user register with the read value could be set by try_handle_mmio if
> try_fwd_ioserv returns IO_HANDLED instead of IO_RETRY.
>
> Is that how the state machine is expected to work?



-- 
Regards,

Oleksandr Tyshchenko