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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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?

~Andrew