Re: [Qemu-devel] [4261] Errors while registering ioports are not fatal (Glauber Costa).

[Anthony Liguori <anthony@codemonkey.ws>]

Paul Brook wrote:
>>> I fail to see how hotplugging or proxing has anything to do with it. IO
>>> port registration is not something that can reasonably fail.
>>>
>>> If the real problem is that we can't cope with multiple devices
>>> registering the same IO port than you need to fix that. Blindly punting
>>> to the caller to
>>>       
>> There is no fix for that.  You can't have two devices that use the same
>> IO port.
>>     
>
> You probably can't meaningfully access the IO port. However I think the 
> registration itself should be allowed.
>
> The device shouldn't have to handle this. Just like real hardware doesn't. 
>   

We can be better than real hardware here.  In the ISA days, having to 
manually configure a cards IRQ lines to avoid conflicts was a huge pain 
for users.  This is not behavior we should strive to emulate :-)

> If you configure two devices with the same IO port I'd expect writes to go to 
> one or both of them, or cause a CPU fault. Just like when you have 
> overlapping memory ranges. The device doesn't suddenly disappear in a puff of 
> smoke because the OS assigned overlapping IO ranges.
>   

The issue isn't the guest OS assigning overlapping IO regions.  The 
issues is that some devices (like IDE), register ioports directly 
independently of PCI regions.

>>> cope is IMHO not an acceptable solution, especially when none of the
>>> callers check the return value.
>>>       
>> IO port range conflicts can still happen even with PCI devices.  Two PCI
>> IDE controllers would conflict with each other for instance.  It's much
>> more likely with ISA of course but it's still possible.  register_ioport
>> really should have a return code and callers should actively be checking
>> it.
>>     
>
> This is why bouncing the error to the device is the wrong thing to do. 
> Once the OS resolves the conflict I'd expect the remaining device to just 
> work.
>   

I don't know how hardware normally handles this.  If this is how normal 
hardware works, then we should emulate that.  It's also true though that 
exit()'ing in register_ioport is the wrong thing to do though.

Regards,

Anthony Liguori

> Paul
>