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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [4261] Errors while registering ioports are not fatal (Glauber Costa).
Date: Sat, 26 Apr 2008 16:09:59 -0500
User-agent: Thunderbird (X11/20080227)

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

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.


Anthony Liguori


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