Re: [PATCH] serial: Fix double migration data

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Marc-André Lureau (marcandre.lureau@gmail.com) wrote:
> Hi
> 
> On Mon, Mar 30, 2020 at 6:47 PM Dr. David Alan Gilbert (git)
> <dgilbert@redhat.com> wrote:
> >
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >
> > After c9808d60281 we have both an object representing the serial-isa
> > device and a separate object representing the underlying common serial
> > uart.  Both of these have vmsd's associated with them and thus the
> > migration stream ends up with two copies of the migration data - the
> > serial-isa includes the vmstate of the core serial.   Besides
> > being wrong, it breaks backwards migration compatibility.
> >
> > Fix this by removing the dc->vmsd from the core device, so it only
> > gets migrated by any parent devices including it.
> > Add a vmstate_serial_mm so that any device that uses serial_mm_init
> > rather than creating a device still gets migrated.
> > (That doesn't fix backwards migration for serial_mm_init users,
> > but does seem to work forwards for ppce500).
> >
> > Fixes: c9808d60281 ('serial: realize the serial device')
> > Buglink: https://bugs.launchpad.net/qemu/+bug/1869426
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  hw/char/serial.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/char/serial.c b/hw/char/serial.c
> > index 2ab8b69e03..c822a9ae6c 100644
> > --- a/hw/char/serial.c
> > +++ b/hw/char/serial.c
> > @@ -1043,7 +1043,6 @@ static void serial_class_init(ObjectClass *klass, void* data)
> >      dc->user_creatable = false;
> >      dc->realize = serial_realize;
> >      dc->unrealize = serial_unrealize;
> > -    dc->vmsd = &vmstate_serial;
> >      device_class_set_props(dc, serial_properties);
> >  }
> >
> > @@ -1113,6 +1112,16 @@ static void serial_mm_realize(DeviceState *dev, Error **errp)
> >      sysbus_init_irq(SYS_BUS_DEVICE(smm), &smm->serial.irq);
> >  }
> >
> > +static const VMStateDescription vmstate_serial_mm = {
> > +    .name = "serial",
> > +    .version_id = 3,
> > +    .minimum_version_id = 2,
> > +    .fields = (VMStateField[]) {
> > +        VMSTATE_STRUCT(serial, SerialMM, 0, vmstate_serial, SerialState),
> > +        VMSTATE_END_OF_LIST()
> > +    }
> > +};
> > +
> 
> Why do you make it a sub-state?

Because it's consistent with serial-isa and it's simple.

> # qemu-system-ppc -M ppce500 -monitor stdio -serial pty
> in 4.2 and 5.0:
>     "serial (8)": {
>         "divider": "0x00d9",
>         "rbr": "0x00",
>         "ier": "0x00",
>         "iir": "0xc1",
>         "lcr": "0x03",
>         "mcr": "0x03",
>         "lsr": "0x60",
>         "msr": "0xb0",
>         "scr": "0x00",
>         "fcr_vmstate": "0x01"
>     },
> 
> With this patch:
>     "serial (8)": {
>         "serial": {
>             "divider": "0x00d9",
>             "rbr": "0x00",
>             "ier": "0x00",
>             "iir": "0xc1",
>             "lcr": "0x03",
>             "mcr": "0x03",
>             "lsr": "0x60",
>             "msr": "0xb0",
>             "scr": "0x00",
>             "fcr_vmstate": "0x01"
>         }
>     },
> 
> >  SerialMM *serial_mm_init(MemoryRegion *address_space,
> >                           hwaddr base, int regshift,
> >                           qemu_irq irq, int baudbase,
> > @@ -1162,6 +1171,7 @@ static void serial_mm_class_init(ObjectClass *oc, void *data)
> >
> >      device_class_set_props(dc, serial_mm_properties);
> >      dc->realize = serial_mm_realize;
> > +    dc->vmsd = &vmstate_serial_mm;
> >  }
> >
> >  static const TypeInfo serial_mm_info = {
> > --
> > 2.25.1
> >
> >
> 
> I understand removing the serial state from SerialClass solves the
> double state issue for ISA. But at the same time, I think we should
> aim to migrate ISASerial state to SerialClass state. I can take a look
> if you want.

I don't think there's anything wrong with having a separate layer here;
the physical reality of what we have is a UART (Serial) that is on the
ISA bus where the ISA bus interfacing doesn't require any extra state
to be migrated.

Dave

> 
> 
> -- 
> Marc-André Lureau
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK