Re: [PATCH 2/4] PCI: endpoint: pci-epf-test: Sanity-check doorbell offset within BAR

[Niklas Cassel <cassel@kernel.org>]

On Mon, Feb 16, 2026 at 12:09:12AM +0900, Koichiro Den wrote:
> pci-epf-test advertises the doorbell target to the RC as a BAR number
> and an offset. The RC rings the doorbell with a single DWORD MMIO write
> to BAR + offset.
> 
> For MSI/MSI-X-based doorbells, the message address is required to be
> DWORD-aligned, so the computed offset should not straddle a BAR boundary
> in normal operation.
> 
> However, with support for doorbells based on mechanisms other than
> MSI/MSI-X (via pci_epf_alloc_doorbell()), the returned message address
> may not necessarily be DWORD-aligned. In such a case, offset plus the
> 32-bit write width could cross the end of the BAR aperture. The offset
> returned by pci_epf_align_inbound_addr() is guaranteed to be within the
> BAR size, but this alone does not ensure that a 32-bit write starting at
> that offset stays within the BAR.
> 
> Add a bounds check to ensure that the 32-bit doorbell write always stays
> within the BAR aperture. While this should not trigger for
> spec-compliant MSI/MSI-X addresses, it provides a defensive guard
> against unexpected offsets from future doorbell implementations.

I think everything you write is true,
and I know that I suggested this...

But for the MMIO address, will it ever not be 32-bit aligned?

Even in the eDMA case, the eDMA registers are 32-bit aligned.

Did I perhaps have a brain fart and overthink this?


I guess theoretically, some future pci_epf_alloc_doorbell() implementation
could return something that is not 32-bit aligned...

But if we really want to add a safety check for that... perhaps a 32-bit
alignment check would be better suited to have in pci_epf_alloc_doorbell() ?


Perhaps this check is better added in pci_epf_alloc_doorbell() or
pci_epf_alloc_doorbell_embedded(), in the series that adds support for
embedded doorbells ?


Kind regards,
Niklas