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

[Stefano Stabellini <sstabellini@kernel.org>]

On Sat, 8 Aug 2020, Oleksandr wrote:
> On 08.08.20 00:50, Stefano Stabellini wrote:
> > On Fri, 7 Aug 2020, Oleksandr wrote:
> > > 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?
> > I think you should check the ioreq->state in try_fwd_ioserv(), if the
> > result is ready, then ioreq->state should be STATE_IORESP_READY, and you
> > can return IO_HANDLED.
> > 
> > That is assuming that you are looking at the live version of the ioreq
> > shared with QEMU instead of a private copy of it, which I am not sure.
> > Looking at try_fwd_ioserv() it would seem that vio->io_req is just a
> > copy? The live version is returned by get_ioreq() ?
> 
> If I understand the code correctly, indeed, get_ioreq() returns live version
> shared with emulator.
> Desired state change (STATE_IORESP_READY) what actually the hvm_wait_for_io()
> is waiting for is set here (in my case):
> https://xenbits.xen.org/gitweb/?p=people/pauldu/demu.git;a=blob;f=demu.c;h=f785b394d0cf141dffa05bdddecf338214358aea;hb=refs/heads/master#l698 
> 
> > Even in handle_hvm_io_completion, instead of setting vio->io_req.state
> > to STATE_IORESP_READY by hand, it would be better to look at the live
> > version of the ioreq because QEMU will have already set ioreq->state
> > to STATE_IORESP_READY (hw/i386/xen/xen-hvm.c:cpu_handle_ioreq).
> It seems that after detecting STATE_IORESP_READY in hvm_wait_for_io() the
> state of live version is set to STATE_IOREQ_NONE immediately, so looking at
> the live version down the handle_hvm_io_completion()
> or in try_fwd_ioserv() shows us nothing I am afraid.

I see. That is because we want to "free" the ioreq as soon as possible,
which is good. handle_hvm_io_completion also sets vio->io_req.state to
STATE_IORESP_READY, so our private copy is still set to
STATE_IORESP_READY. Thus, try_fwd_ioserv should do the right thing
simply reading vio->io_req.state: try_fwd_ioserv should be able to
return IO_HANDLED when the "state" is STATE_IORESP_READY, right?