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

[Markus Armbruster <armbru@redhat.com>]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> On Tue, Apr 27, 2021 at 02:02:27PM -0400, John Snow wrote:
>> On 4/27/21 1:54 PM, 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.
>> > 
>> > Regards,
>> > 
>> > Phil.
>> > 
>> 
>> It sounds complicated. In the meantime, I think I am favor of taking
>> Thomas's patch because it merely adds some error routing to allow us to
>> avoid a crash. The core organizational issues of the IDE device(s) will
>> remain and can be solved later as needed.
>
> The approach in this patch is okay but we should keep in mind it only
> solves piix3-ide. ISA provides a non-qdev backdoor API and there may be
> more instances of this type of bug.
>
> A qdev fix would address the root cause and make it possible to drop the
> backdoor API, but that's probably too much work for little benefit.

What do you mean by backdoor API?  Global @isabus?