Re: [PATCH] memory: Do not allow subregion out of the parent region range

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/12/19 19:17, Peter Maydell wrote:
> On Tue, 17 Dec 2019 at 16:57, Richard Henderson
> <richard.henderson@linaro.org> wrote:
>>
>> On 12/17/19 1:58 AM, Christophe de Dinechin wrote:
>>>
>>>
>>>> On 17 Dec 2019, at 11:51, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>>> Yes, the idea is that you could have for one version of the device
>>>>
>>>>   parent 0x000-0x7ff
>>>>     stuff 0x000-0x3ff
>>>>     morestuff 0x400-0x7ff
>>>>
>>>> and for another
>>>>
>>>>   parent 0x000-0x3ff
>>>>     stuff 0x000-0x3ff
>>>>     morestuff 0x400-0x7ff
>>>>
>>>> where parent is the BAR, and you can share the code to generate the tree
>>>> underneath parent.
>>>
>>> I can see why you would have code reuse reasons to do that,
>>> but frankly it looks buggy and confusing. In the rare cases
>>> where this is indented, maybe add a flag making it explicit?
>>
>> The guest OS is programming the BAR, producing a configuration that, while it
>> doesn't make sense, is also legal per PCI.  QEMU cannot abort for this
>> configuration.
> 
> Does guest programming of the PCI BAR size actually change the size
> of the 'parent' region, or does it just result in the creation
> of an appropriately sized alias into 'parent' ?

Resizable BARs are not handled by the PCI host bridge but rather from
the device itself, so the device is free to handle them either way.

Paolo