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Re: [PATCH v2 2/4] PCI: determine whether a device has extended config space
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
- Date: Fri, 23 Jan 2026 17:24:31 -0500
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 23 Jan 2026 22:25:06 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 1/19/26 09:46, Jan Beulich wrote:
> Legacy PCI devices don't have any extended config space. Reading any part
> thereof may return all ones or other arbitrary data, e.g. in some cases
> base config space contents repeatedly.
>
> Logic follows Linux 6.19-rc's pci_cfg_space_size(), albeit leveraging our
> determination of device type; in particular some comments are taken
> verbatim from there.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
> ---
> Should we skip re-evaluation when pci_mmcfg_arch_enable() takes its early
> exit path?
I don't have a strong opinion here, though I'm leaning toward it's OK as is.
>
> The warning near the bottom of pci_check_extcfg() may be issued multiple
> times for a single device now. Should we try to avoid that?
If I'm reading Linux drivers/xen/pci.c correctly, my understanding is that dom0
will only invoke PHYSDEVOP_pci_mmcfg_reserved once per mmcfg segment, so in
practice it's unlikely to spam. In other words, I think it's OK as is.
>
> Note that no vPCI adjustments are done here, but they're going to be
> needed: Whatever requires extended capabilities will need re-
> evaluating / newly establishing / tearing down in case an invocation of
> PHYSDEVOP_pci_mmcfg_reserved alters global state.
Agreed. The current patch is prerequisite for this work. Hm, perhaps we could
create a gitlab ticket for it?
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