Re: [PATCH] hw/ide: Fix crash when plugging a piix3-ide device into the x-remote machine

[Markus Armbruster]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> On Tue, Apr 27, 2021 at 07:54:21PM +0200, Philippe Mathieu-Daudé wrote:
>> On 4/27/21 7:16 PM, John Snow wrote:
>> > On 4/27/21 9:54 AM, Stefan Hajnoczi wrote:
>> >> I suggest fixing this at the qdev level. Make piix3-ide have a
>> >> sub-device that inherits from ISA_DEVICE so it can only be instantiated
>> >> when there's an ISA bus.
>> >>
>> >> Stefan
>> > 
>> > My qdev knowledge is shaky. Does this imply that you agree with the
>> > direction of Thomas's patch, or do you just mean to disagree with Phil
>> > on his preferred course of action?
>> 
>> My understanding is a disagreement to both, with a 3rd direction :)
>> 
>> I agree with Stefan direction but I'm not sure (yet) that a sub-device
>> is the best (long-term) solution. I guess there is a design issue with
>> this device, and would like to understanding it first.
>> 
>> IIUC Stefan says the piix3-ide is both a PCI and IDE device, but QOM
>> only allow a single parent. Multiple QOM inheritance is resolved as
>> interfaces, but PCI/IDE qdev aren't interfaces, rather abstract objects.
>> So he suggests to embed an IDE device within the PCI piix3-ide device.
>> 
>> My view is the PIIX is a chipset that share stuffs between components,
>> and the IDE bus belongs to the chipset PCI root (or eventually the
>> PCI-ISA bridge, function #0). The IDE function would use the IDE bus
>> from its root parent as a linked property.
>> My problem is currently this device is user-creatable as a Frankenstein
>> single PCI function, out of its chipset. I'm not sure yet this is a
>> dead end or I could work something out.
>
> Kevin and Paolo previously pointed out that piix3-ide is sometimes used
> with the Q35 machine type. The user-creatable piix3-ide device needs to
> be deprecated before it can be dropped. That's a long process that
> cannot fix the current crash any time soon.
>
> I do support deprecating the user-creatable piix3-ide device in favor of
> a proper Q35 Legacy IDE implementation. The main problem is this
> involves a bunch of work and I'm not sure who would do it (the payoff is
> not very high).

In my opinion, letting users plug device models for PCI *functions* as
if they were *devices* was a mistake.  Compounding the mistake of not
modelling the difference between PCI function and PCI device.

The more of them we can deprecate, the better.