[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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




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




 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.