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RE: [PATCH for-4.17 v3 1/2] pci: do not disable memory decoding for devices
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Henry Wang <Henry.Wang@xxxxxxx>
- Date: Fri, 28 Oct 2022 02:56:12 +0000
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Paul Durrant <paul@xxxxxxx>
- Delivery-date: Fri, 28 Oct 2022 02:56:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH for-4.17 v3 1/2] pci: do not disable memory decoding for devices
Hi Roger,
> -----Original Message-----
> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Subject: [PATCH for-4.17 v3 1/2] pci: do not disable memory decoding for
> devices
>
> Commit 75cc460a1b added checks to ensure the position of the BARs from
> PCI devices don't overlap with regions defined on the memory map.
> When there's a collision memory decoding is left disabled for the
> device, assuming that dom0 will reposition the BAR if necessary and
> enable memory decoding.
>
> While this would be the case for devices being used by dom0, devices
> being used by the firmware itself that have no driver would usually be
> left with memory decoding disabled by dom0 if that's the state dom0
> found them in, and thus firmware trying to make use of them will not
> function correctly.
>
> The initial intent of 75cc460a1b was to prevent vPCI from creating
> MMIO mappings on the dom0 p2m over regions that would otherwise
> already have mappings established. It's my view now that we likely
> went too far with 75cc460a1b, and Xen disabling memory decoding of
> devices (as buggy as they might be) is harmful, and reduces the set of
> hardware on which Xen works.
>
> This commits reverts most of 75cc460a1b, and instead adds checks to
> vPCI in order to prevent misplaced BARs from being added to the
> hardware domain p2m. Signaling on whether BARs are mapped is tracked
> in the vpci structure, so that misplaced BARs are not mapped, and thus
> Xen won't attempt to unmap them when memory decoding is disabled.
>
> This restores the behavior of Xen for PV dom0 to the state it was
> previous to 75cc460a1b, while also introducing a more contained fix
> for the vPCI BAR mapping issues.
>
> Fixes: 75cc460a1b ('xen/pci: detect when BARs are not suitably positioned')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Release-acked-by: Henry Wang <Henry.Wang@xxxxxxx>
Kind regards,
Henry
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