Re: [PATCH] x86/PVH: PHYSDEVOP_pci_mmcfg_reserved should not blindly register a region

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.05.2020 14:54, Andrew Cooper wrote:
> On 08/05/2020 13:43, Jan Beulich wrote:
>> The op has a register/unregister flag, and hence registration shouldn't
>> happen unilaterally. Introduce unregister_vpci_mmcfg_handler() to handle
>> this case.
>>
>> Fixes: eb3dd90e4089 ("x86/physdev: enable PHYSDEVOP_pci_mmcfg_reserved for 
>> PVH Dom0")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I agree in principle that registration shouldn't be unilateral, but why
> on earth does the API behave like that to begin with?
> 
> There is no provision to move or update MMCFG regions in any spec I'm
> aware of, and hardware cannot in practice update memory routing like this.
> 
> Under what circumstances should we tolerate an unregister in the first
> place?

Hot unplug of an entire segment, for example.

Jan