Re: [Qemu-devel] [PATCH v2 0/3] PCDIMM cleanup

[Wei Yang]

On Wed, Feb 27, 2019 at 06:27:49PM +0100, Igor Mammedov wrote:
>On Wed, 27 Feb 2019 13:59:20 +0000
>Wei Yang <address@hidden> wrote:
>
>> On Wed, Feb 27, 2019 at 02:12:42PM +0100, Igor Mammedov wrote:
>> >On Mon, 25 Feb 2019 12:47:14 +0000
>> >Wei Yang <address@hidden> wrote:
>> >  
>> >> On Mon, Feb 25, 2019 at 09:05:37AM +0100, Igor Mammedov wrote:  
>> >> >On Sat, 23 Feb 2019 00:02:49 +0000
>> >> >Wei Yang <address@hidden> wrote:
>> >> >    
>> >> >> On Thu, Feb 21, 2019 at 03:50:04PM +0100, Igor Mammedov wrote:    
>> >> >> >On Wed, 20 Feb 2019 08:51:21 +0800
>> >> >> >Wei Yang <address@hidden> wrote:
>> >> >> >    
>> >> >> >> Three trivial cleanup for pc-dimm.
>> >> >> >> 
>> >> >> >> Patch [1] remove the check on class->hotpluggable since pc-dimm is 
>> >> >> >> always
>> >> >> >> hotpluggable.
>> >> >> >> Patch [2] remove nvdimm_realize
>> >> >> >> Patch [2] remove pcdimm realize-callback    
>> >> >> >even though this series doesn't break anything, I disagree with it
>> >> >> >conceptually as it makes device less abstracted and make it more
>> >> >> >dependent on how existing machine code uses it.
>> >> >> >I'd drop whole series.
>> >> >> >    
>> >> >> 
>> >> >> Is Patch [1] also make device more dependent on existing 
>> >> >> implementation?    
>> >> >yes, it's implementation detail that PCDIMM&Co are hotpluggable now.
>> >> >    
>> >> >> For example, when we look at the counterpart of acpi_memory_plug_cb():
>> >> >> 
>> >> >>     acpi_pcihp_device_plug_cb
>> >> >> 
>> >> >> which handle the pci device hotplug. We don't check the hotpluggable
>> >> >> property for pci devices.
>> >> >> 
>> >> >> To me, this is a general rule for PCDIMM, they are hotpluggable.    
>> >> >yes, PCDIMMs are hotpluggable but apci device (piix4pm/ich9/...) 
>> >> >handling hotplug
>> >> >should ignore any non-hotpluggable one. If you remove check and then 
>> >> >later
>> >> >someone else would add non-hotpluggable memory device or make PC-DIMMs 
>> >> >or its
>> >> >variant of non-hotpluggable one, acpi device handling will break.
>> >> >Hence I'd prefer to keep the check.  
>> >> >     
>> >> 
>> >> Ok, if we'd support un-hotpluggable mem device, we could retain this
>> >> check. But this maybe not a proper place.
>> >> 
>> >> Based on my understanding, at this point, every thing has been done
>> >> properly. If this check pass, we will send an acpi interrupt to notify
>> >> the guest. In case this is an un-hotpluggable device, every thing looks
>> >> good but no effect in guest. Because we skip the notification.
>> >> 
>> >> Maybe we need to move the check in pre-plug?  
>> >And what would happen then and afterwards?
>> >
>> >Point is to make one of the handlers in chain to ignore plug event
>> >(i.e. do not generate SCI event) while the rest of handlers complete
>> >successfully mapping device in address space and whatever else.
>> >  
>> 
>> This will have a well prepared device in system but guest is not
>> notified, right?
>yes, it's theoretically possible to move check up the call stack
>to machine level and not call secondary hotplug handler on non hotplugged
>device but that again depends on what secondary hotplug handler does.
>I'd rather keep logic independent here unless there is good reason not
>to do so.
>
>
>> My idea to move the check in pre-plug will result the qemu return when
>> it see no SCI event will be generated, so there is no device created.
>> 
>> I guess current behavior is a designed decision?
>I'd say so.
>PS:
>QEMU's hotplug_hadler API is misnamed at this point as it handles both
>cold (basic device wiring) and hotplug (processing hotplug).
>Maybe we should rename it but I'm not irritated enough by it to do so.
>

Got it, thanks.

>> >> >> For Patch[2][3], I agree with you.
>> >> >>     
>> >>   
>> 

-- 
Wei Yang
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