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

[Stefan Hajnoczi]

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.

Stefan

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