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Re: [PATCH v1 0/2] x86/pci: MMCFG improvements and always use it if available
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Wed, 7 Jan 2026 18:22:05 +0100
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Jan Beulich <jbeulich@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Delivery-date: Wed, 07 Jan 2026 17:22:22 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
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