Re: [Qemu-devel] [PATCH 01/15] pc: merge pc_piix.c into pc.c

[Jan Kiszka]

On 2012-01-27 14:07, Anthony Liguori wrote:
> On 01/27/2012 02:50 AM, Jan Kiszka wrote:
>> On 2012-01-26 20:00, Anthony Liguori wrote:
>>> A long time ago, there was a grand plan to merge q35 chipset support.  The 
>>> start
>>> of that series was a refactoring of pc.c which split a bunch of the "common"
>>> functionality into a separate file that could be shared by the two.
>>>
>>> But q35 never got merged and the refactoring, in retrospect, just made 
>>> things
>>> worse.  Making things proper objects and using composition is the right way
>>> to share common devices.
>>>
>>> By pulling these files back together, we can start to fix some of this mess.
>>
>> There are surely things to clean up and improve, but a clear NACK for
>> the general direction.
> 
> Hi Jan,
> 
> I think you're missing the bigger picture here.  Once this refactoring 
> finishes, 
> here's what we'll be left with:
> 
> 1) pc_init creates an I440FX, any bus devices (ISA serial port, PCI vga and 
> nics, etc.), sets properties appropriately, and realizes the devices.
> 
> 2) I440FX is-a PCIHost, has-a I440FX-PMC, has-a PIIX3
> 
> 3) PIIX3 has-a RTC, has-a I8042, has-a DMAController, etc.
> 
> Memory creation is done by the I440FX-PMC.
> 
> Now it's true that a newer chipset is going to be similar.  It will likely 
> have 
> a SuperI/O chip that looks similar to PIIX3.  The right way to share code 
> would 
> be to move most of the PIIX3 functionality to a base class (PCSuperIO) that 
> PIIX3 inherits from.
> 
> This is probably how to support ISAPC properly too.

The ISAPC is differently composed. The board creates all those
individual chips that are otherwise part of the SuperIO block of the
chipset. And IRQ wiring is different. So, no, I don't think it is the
right model. I would rather think of a pc_isa.c that does the proper
composing.

>  Once I sort out interrupts, 
> I'll attempt to tackle that.  My guess is that a SuperIO chip could be an 
> ISADevice and that we could simply make the PIIX3 has-a SuperIO.  Then the 
> ISAPC 
> would have a trivial ISA chipset that has-a SuperIO.
> 
> This is fairly trivial to do once we have the right structure to the code.
> 
> But the current code has the wrong structure which is why there's so much 
> pointer chasing and passing.

Just look at your code and count the generic, PIIX3-independent
functions. Keep them in pc.c, move the rest to pc_piix.c. You could try
to model the ISA accordingly. I think some pc_isa.c would help to
establish a good split-up already now, in the absence of a third chipset.

> 
>>
>> It's undoubted that we need a more modern chipset than this ancient
>> PIIX3, rather sooner than later. And it is clear that there is a good
>> amount of generic functions in pc.c for building a PC, even a fairly
>> modern one. So we need a common lib for PC chipsets and would only
>> revert what you start here.
> 
> Sorry, but I don't view this as a useful requirement.  Today we support two 
> types of PCs: an i440fx based system and an ISA-only system.  We should 
> concentrate on modeling those two systems in the most natural way sharing as 
> much code as possible.

A modern chipset is the only sane way to add things like PCIe,
hotplugging, power management, etc., and to enable standard PC
components like AHCI or EHCI/xHCI by default.

Jan

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