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Date: Wed, 25 Aug 2010 11:49:19 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] qdev: Reset hotplugged devices
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To: Isaku Yamahata <yamahata@valinux.co.jp>
Cc: Alex Williamson <alex.williamson@redhat.com>, glommer@redhat.com, Markus Armbruster <armbru@redhat.com>, qemu-devel@nongnu.org

On 08/25/2010 10:17 AM, Isaku Yamahata wrote:
> On Wed, Aug 25, 2010 at 07:55:27AM -0500, Anthony Liguori wrote:
>    
>>> Maybe we can merge the patches.
>>> As for your patch, I have some comment.
>>> - bus itself may want its own handler. At lease pci bus needs it.
>>>     And propagating reset signal to children is up to the bus controller.
>>>
>>>        
>> I disagree.  Reset should be equivalent to power off + init and it's not
>> something that can be selectively propagated.
>>
>> There are different types of bus level resets and it may make sense to
>> model that in the PCI layer but the qdev reset is a power cycle
>> semantically.
>>
>> I think using a walker pattern is a stronger design than having each
>> reset function do it's own transversal because the later has the
>> potential to introduce bugs.
>>      
> Warm reset is different from cold reset, so reset isn't equivalent to
> a power cycle.

But qdev reset is not warm reset.  It's cold reset.  IOW, it's called 
when the device is first powered up.

>   Typically registers which report errors must retain the
> contents across warm reset, but doesn't across a power cycle.
> I can see the reset call back is called when both power on and warm reset,
> but I don't see why qdev_reset() is a power cycle semantically.
>
> Although your current patch might work for me at the moment,
> the claim that qdev_reset() == a power cycle worries me.
>
> How about adding a argument to distinguish reset type?
> Something like QEMU_RESET_COLD, QEMU_RESET_WARM.
>    

Before approaching this, I think we ought to better understand exactly 
what type of reset semantics we would need to support in the future.

I think that starts by understanding exactly what's guaranteed and 
understanding what the use cases are for it.

Regards,

Anthony Liguori