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.
