Re: [PATCH v1 0/2] x86/pci: MMCFG improvements and always use it if available

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Jan 07, 2026 at 04:54:55PM +0000, Teddy Astie wrote:
> Currently, Xen uses legacy method to access the configuration space unless the
> access cannot be made with it, where Xen fallbacks to MMCFG. This is not really
> great, as MMCFG is more flexible and doesn't require a dedicated lock, so it would
> be preferable to use it whenever possible.
> 
> Teddy Astie (2):
>   x86/pci: Improve pci_mmcfg_{read,write} error handling
>   x86/pci: Prefer using mmcfg for accessing configuration space

AFAICT Linux is using the same approach as Xen to perform PCI
accesses.  Registers below 256 on segment 0 are accessed using the
legacy method (IO ports), while the extended space is accessed using
MMCFG.  Do you know the reason for this?  I fear there might be
legacy devices/bridges (or root complexes?) where MMCFG is not
working as expected?

I think we need to understand why Xen (and Linux) do it this way so it
can be properly justified why it's safe to switch to a different
approach.

Thanks, Roger.