Re: [Qemu-devel] [PATCHv7 5/6] fw_cfg: move qdev_init_nofail() from fw_cfg_init1() to callers

["Eduardo Habkost" <ehabkost@redhat.com>]

On Fri, Jul 07, 2017 at 01:33:20PM +0200, Igor Mammedov wrote:
> On Tue, 4 Jul 2017 19:08:44 +0100
> Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> wrote:
> 
> > On 03/07/17 10:39, Igor Mammedov wrote:
> > 
> > > On Thu, 29 Jun 2017 15:07:19 +0100
> > > Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> wrote:
> > >   
> > >> When looking to instantiate a TYPE_FW_CFG_MEM or TYPE_FW_CFG_IO device to be
> > >> able to wire it up differently, it is much more convenient for the caller to
> > >> instantiate the device and have the fw_cfg default files already preloaded
> > >> during realize.
> > >>
> > >> Move fw_cfg_init1() to the end of both the fw_cfg_mem_realize() and
> > >> fw_cfg_io_realize() functions so it no longer needs to be called manually
> > >> when instantiating the device, and also rename it to fw_cfg_common_realize()
> > >> which better describes its new purpose.
> > >>
> > >> Since it is now the responsibility of the machine to wire up the fw_cfg device
> > >> it is necessary to introduce a object_property_add_child() call into
> > >> fw_cfg_init_io() and fw_cfg_init_mem() to link the fw_cfg device to the root
> > >> machine object as before.
> > >>
> > >> Finally we can now convert the asserts() preventing multiple fw_cfg devices
> > >> and unparented fw_cfg devices being instantiated and replace them with proper
> > >> error reporting at realize time. This allows us to remove FW_CFG_NAME and
> > >> FW_CFG_PATH since they are no longer required.
> > >>
> > >> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> > >> ---
> > >>  hw/nvram/fw_cfg.c |   41 +++++++++++++++++++++++++++++------------
> > >>  1 file changed, 29 insertions(+), 12 deletions(-)
> > >>
> > >> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> > >> index 2291121..31029ac 100644
> > >> --- a/hw/nvram/fw_cfg.c
> > >> +++ b/hw/nvram/fw_cfg.c
> > >> @@ -37,9 +37,6 @@
> > >>  
> > >>  #define FW_CFG_FILE_SLOTS_DFLT 0x20
> > >>  
> > >> -#define FW_CFG_NAME "fw_cfg"
> > >> -#define FW_CFG_PATH "/machine/" FW_CFG_NAME
> > >> -
> > >>  #define TYPE_FW_CFG     "fw_cfg"
> > >>  #define TYPE_FW_CFG_IO  "fw_cfg_io"
> > >>  #define TYPE_FW_CFG_MEM "fw_cfg_mem"
> > >> @@ -920,19 +917,22 @@ static int fw_cfg_unattached_at_realize(void)
> > >>  }
> > >>  
> > >>  
> > >> -static void fw_cfg_init1(DeviceState *dev)
> > >> +static void fw_cfg_common_realize(DeviceState *dev, Error **errp)
> > >>  {
> > >>      FWCfgState *s = FW_CFG(dev);
> > >>      MachineState *machine = MACHINE(qdev_get_machine());
> > >>      uint32_t version = FW_CFG_VERSION;
> > >>  
> > >> -    assert(!object_resolve_path(FW_CFG_PATH, NULL));
> > >> -
> > >> -    object_property_add_child(OBJECT(machine), FW_CFG_NAME, OBJECT(s), NULL);
> > >> -
> > >> -    qdev_init_nofail(dev);
> > >> +    if (!fw_cfg_find()) {  
> > > maybe add comment that here, that fw_cfg_find() will return NULL if more than
> > > 1 device is found.  
> > 
> > This should be the same behaviour as before, i.e. a NULL means that the
> > fw_cfg device couldn't be found. It seems intuitive to me from the name
> > of the function exactly what it does, so I don't find the lack of
> > comment too confusing. Does anyone else have any thoughts here?
> intuitive meaning from the function name would be return NULL if nothing were found,
> however it's not the case here.
> 
> taking in account that fwcfg in not user creatable device how about:
> 
> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> index 316fca9..8f6eef8 100644
> --- a/hw/nvram/fw_cfg.c
> +++ b/hw/nvram/fw_cfg.c
> @@ -1014,7 +1014,10 @@ FWCfgState *fw_cfg_init_mem(hwaddr ctl_addr, hwaddr data_addr)
>  
>  FWCfgState *fw_cfg_find(void)
>  {
> -    return FW_CFG(object_resolve_path(FW_CFG_PATH, NULL));
> +    bool ambig = false;
> +    FWCfgState *f = FW_CFG(object_resolve_path_type("", TYPE_FW_CFG, &ambig));
> +    assert(!ambig);
> +    return f;
>  }
> 
> or if we must to print user friendly error and fail realize gracefully
> (not sure why) just add errp argument to function so it could report
> error back to caller, then places that do not care much about graceful
> exit would use error_abort as argument and realize would use
> its local_error/errp argument.

I don't disagree with adding the assert(), but it looks like
making fw_cfg_find() return NULL if there are multiple devices
can be useful for realize.

In this case, it looks like Mark is relying on that in
fw_cfg_common_realize(): if multiple devices are created,
fw_cfg_find() will return NULL, and realize will fail.  This
sounds like a more graceful way to handle multiple-device
creation than crashing on fw_cfg_find().  This is the solution
used by find_vmgenid_dev()/vmgenid_realize(), BTW.

Either way, we have to choose: either we make fw_cfg_find() crash
when multiple devices exist (in this case, the fw_cfg_find() call
on realize will be useless), or we make it return NULL and
document it very clearly.


> 
>    
> > >> +        error_setg(errp, "at most one %s device is permitted", TYPE_FW_CFG);  
> > > s/TYPE_FW_CFG/object_get_typename()/
> > > so it would print leaf type name   

I disagree.  We allow at most one fw_cfg device (it doesn't
matter which type), not at most one device of each leaf type.
Saying "at most one fw_cfg_mem device is permitted" if 1
fw_cfg_io and 1 fw_cfg_mem device is created would be misleading.


> > 
> > Previously the code would add the device to the machine at FW_CFG_PATH
> > which was fixed at /machine/fw_cfg regardless of whether it was
> > fw_cfg_io or fw_cfg_mem type (see the top of fw_cfg.c).
> > 
> > This was a deliberate attempt to preserve the existing behaviour and if
> > we were to give the leaf type name I think this would be misleading,
> > since it implies you could have one fw_cfg_io and one fw_cfg_mem which
> > isn't correct.
> I don't have strong preference here, considering that it's
> hardcoded in board (not user specified) device,
> developer that stumbles upon error should be able to dig out which
> concrete type caused it.
>    
> > >> +        return;
> > >> +    }
> > >>  
> > >> -    assert(!fw_cfg_unattached_at_realize());
> > >> +    if (fw_cfg_unattached_at_realize()) {  
> > > as I pointed out in v6, this condition will always be false,
> > > I suggest to drop 4/6 patch and this hunk here so it won't to confuse
> > > readers with assumption that condition might succeed.  
> > 
> > Definitely look more closely at the fw_cfg_unattached_at_realize()
> > implementation in patch 4. You'll see that the check to determine if the
> > device is attached does not check obj->parent but checks to see if the
> > device exists under /machine/unattached which is what the
> > device_set_realised() does if the device doesn't have a parent.
> looking more fw_cfg_unattached_at_realize(),
>  returns true if fwcfg is direct child of/machine/unattached
> meaning implicit parent assignment.
> 
> As result, above condition essentially forbids having fwcfg under
> /machine/unattached and forces user to explicitly set parent
> outside of /machine/unattached or be a child of some other device.
> 
> Is this your intent (why)?

I'm confused by this part as well.  I still don't see the point
of fw_cfg_unattached_at_realize(), I need to re-read the patches
and commit messages to try to understand that.

If we are changing fw_cfg_find() to not care about the fw_cfg
device location, we don't need to care if it's in
/machine/unattached or not.

> [...]

-- 
Eduardo