Re: [Qemu-devel] [PATCH 1/2] Allow QEMUMachine to override reset sequencing

[Anthony Liguori <anthony@codemonkey.ws>]

David Gibson <david@gibson.dropbear.id.au> writes:

> At present the qemu_system_reset() function always performs the same basic
> actions on all machines.  This includes running all the reset handler
> hooks, however the order in which they run is not controlled by the board
> logic.

Let's be careful here in referring to "board logic".

Reset should propagate--there's no argument there.  Having all devices
register in a list with no control over how reset is dispatched is
wrong.  It's workable because of qdev imposes per-bus reset functions
and typically we have a small number of busses and ordering doesn't
matter.

> This is incorrect: any modern real hardware will generally have some sort
> of reset controller, sometimes a full microcontroller or even service
> processor which will control the order in which components on the board are
> reset.

I think this is true only with a loose definition of "modern", but okay.

I don't think this is the right argument.  The machine should serve as
the entry point for reset.  What components get reset in what order will
need to be a machine hook for practical purposes (since not everything
will be fully QOMized all at once).

So I think this is the right approach for now.

But all of the DT initialization stuff that is leading to this
discussion in the first place is a gross hack to make a PV architecture
work that took far too many short cuts.

Building a DT in memory representing hardware instead of making things
discoverable is not how hardware works.  This sort of stuff is either
(1) hard coded in a firmware/flashrom or (2) built dynamically in
firmware.  Let's not pretend like we're doing this because it's needed
for real hardware.

Regards,

Anthony Liguori

> For one specific example, the machine level reset handlers may need to
> re-establish certain things in memory to meet the emulated platform's
> specified entry conditions, before reexecuting the guest software.
> re-establish the correct specified entry conditions for the emulated
> platform.  This must happen _after_ resetting peripheral devices, or they
> could still be in the middle of DMA operations which would clobber the new
> memory content.  However with the normal ordering of reset hooks, the
> machine is not able to register a hook which will run after the normal
> qdev reset registered by generic code.
>
> This patch allows the machine to take control of the sequence of operations
> during reset, by adding a new reset hook to the QEMUMachine.  This hook is
> optional - without it we just use the same reset sequence as always.  That
> logic is available in qemu_default_system_reset() to make it easy to
> implement the case of the machine logic just needing to do some things
> either before or after the normal reset.
>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  hw/boards.h |    3 +++
>  sysemu.h    |    1 +
>  vl.c        |   10 +++++++++-
>  3 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/hw/boards.h b/hw/boards.h
> index 59c01d0..f2bfd3e 100644
> --- a/hw/boards.h
> +++ b/hw/boards.h
> @@ -12,11 +12,14 @@ typedef void QEMUMachineInitFunc(ram_addr_t ram_size,
>                                   const char *initrd_filename,
>                                   const char *cpu_model);
>  
> +typedef void QEMUMachineResetFunc(bool report);
> +
>  typedef struct QEMUMachine {
>      const char *name;
>      const char *alias;
>      const char *desc;
>      QEMUMachineInitFunc *init;
> +    QEMUMachineResetFunc *reset;
>      int use_scsi;
>      int max_cpus;
>      unsigned int no_serial:1,
> diff --git a/sysemu.h b/sysemu.h
> index 6540c79..f74319b 100644
> --- a/sysemu.h
> +++ b/sysemu.h
> @@ -62,6 +62,7 @@ int qemu_powerdown_requested(void);
>  void qemu_system_killed(int signal, pid_t pid);
>  void qemu_kill_report(void);
>  extern qemu_irq qemu_system_powerdown;
> +void qemu_default_system_reset(bool report);
>  void qemu_system_reset(bool report);
>  
>  void qemu_add_exit_notifier(Notifier *notify);
> diff --git a/vl.c b/vl.c
> index 9fea320..ac47a7c 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1396,7 +1396,7 @@ void qemu_unregister_reset(QEMUResetHandler *func, void *opaque)
>      }
>  }
>  
> -void qemu_system_reset(bool report)
> +void qemu_default_system_reset(bool report)
>  {
>      QEMUResetEntry *re, *nre;
>  
> @@ -1410,6 +1410,14 @@ void qemu_system_reset(bool report)
>      cpu_synchronize_all_post_reset();
>  }

Not a huge fan of the naming here.  How about qemu_devices_reset()?

> +void qemu_system_reset(bool report)
> +{
> +    if (current_machine->reset)
> +        current_machine->reset(report);
> +    else
> +        qemu_default_system_reset(report);
> +}

Missing curly braces.

Regards,

Anthony Liguori

> +
>  void qemu_system_reset_request(void)
>  {
>      if (no_reboot) {
> -- 
> 1.7.10.4