Re: [PATCH v4 00/12] PCI: endpoint: pci-epf-vntb / NTB: epf: Enable per-doorbell bit handling

[Dave Jiang]

On 5/20/26 11:29 PM, Koichiro Den wrote:
> On Tue, May 19, 2026 at 02:29:51PM -0700, Dave Jiang wrote:
>>
>>
>> On 5/12/26 7:49 PM, Koichiro Den wrote:
>>> This series fixes doorbell bit/vector handling for the EPF-based NTB
>>> pair (ntb_hw_epf <-> pci-epf-*ntb). Its primary goal is to enable safe
>>> per-db-vector handling in the NTB core and clients (e.g. ntb_transport),
>>> without changing the on-the-wire doorbell mapping.
>>>
>>>
>>> Background / problem
>>> ====================
>>>
>>> ntb_hw_epf historically applies an extra offset when ringing peer
>>> doorbells: the link event uses the first interrupt slot, and doorbells
>>> start from the third slot (i.e. a second slot is effectively unused).
>>> pci-epf-vntb carries the matching offset on the EP side as well.
>>>
>>> As long as db_vector_count()/db_vector_mask() are not implemented, this
>>> mismatch is mostly masked. Doorbell events are effectively treated as
>>> "can hit any QP" and the off-by-one vector numbering does not surface
>>> clearly.
>>>
>>> However, once per-vector handling is enabled, the current state becomes
>>> problematic:
>>>
>>>   - db_valid_mask exposes bits that do not correspond to real doorbells
>>>     (link/unused slots leak into the mask).
>>>   - ntb_db_event() is fed with 1-based/shifted vectors, while NTB core
>>>     expects a 0-based db_vector for doorbells.
>>>   - On pci-epf-vntb, .peer_db_set() may be called in atomic context, but
>>>     it directly calls pci_epc_raise_irq(), which can sleep.
>>>
>>>
>>> Why NOT fix the root offset?
>>> ============================
>>>
>>> The natural "root" fix would be to remove the historical extra offset in
>>> the peer_db_set() doorbell paths for ntb_hw_epf and pci-epf-vntb.
>>> Unfortunately this would lead to interoperability issues when mixing old
>>> and new kernel versions (old/new peers). A new side would ring a
>>> different interrupt slot than what an old peer expects, leading to
>>> missed or misrouted doorbells, once db_vector_count()/db_vector_mask()
>>> are implemented.
>>>
>>> Therefore this series intentionally keeps the legacy offset, and instead
>>> fixes the surrounding pieces so the mapping is documented and handled
>>> consistently in masks, vector numbering, and per-vector reporting.
>>>
>>>
>>> Dependencies
>>> ============
>>>
>>> This v4 is prepared on top of the current pci/endpoint:
>>> commit f5aa97125491 ("misc: pci_endpoint_test: remove dead BAR read before
>>>                       doorbell trigger")
>>
>> I can't seem to apply the patch set via 'b4 shazam' on top of f5aa97125491. Trying to set it up for review.
> 
> Hi Dave,
> 
>>
>> DJ
>>
>>>
>>> plus these patches as contextual prerequisites:
>>>
>>>   - [PATCH 0/2] NTB: epf: Fix ntb_hw_epf ISR issues
>>>     https://lore.kernel.org/ntb/20260304083028.1391068-1-den@xxxxxxxxxxxxx/
>>>
>>>   - [PATCH 0/2] PCI: endpoint: pci-epf-{v}ntb: A couple of fixes
>>>     https://lore.kernel.org/linux-pci/20260407124421.282766-1-mani@xxxxxxxxxx/
>>>
>>> (These touch the same code part as this series and would otherwise conflict
>>> with it. They should land upstream first in my opinion.)
> 
> This series depends on the two prerequisite series listed above.

They don't apply cleanly either. Do you have a public repo somewhere that has everything applied? That may be the easiest way to do things if a series has complicated dependencies.

DJ

> 
> Best regards,
> Koichiro
> 
>>>
>>>
>>> Practical benefit
>>> =================
>>>
>>> This also enables ntb_netdev's multi-queue performance scale:
>>>
>>>   [PATCH v3 0/4] net: ntb_netdev: Add Multi-queue support
>>>   https://lore.kernel.org/netdev/20260305155639.1885517-1-den@xxxxxxxxxxxxx/
>>>
>>> Each netdev queue maps to an ntb_transport queue pair. With correct
>>> db_vector_count() and db_vector_mask() callbacks, ntb_transport can wake
>>> only the queue pairs covered by the doorbell vector it received. Without
>>> those callbacks, the NTB core falls back to the full valid doorbell mask,
>>> so each doorbell interrupt can still wake all queue pairs. See "Testing /
>>> Results" section in the above link.
>>>
>>>
>>> What this series does
>>> =====================
>>>
>>> - pci-epf-vntb:
>>>
>>>   - Document the legacy offset.
>>>   - Defer MSI doorbell raises to process context to avoid sleeping in
>>>     atomic context. This becomes relevant once multiple doorbells are
>>>     raised concurrently at a high rate.
>>>   - Report doorbell vectors as 0-based to ntb_db_event().
>>>   - Reject unusable doorbell counts and guard configfs writes after EPC
>>>     attach with -EOPNOTSUPP.
>>>   - Fix db_valid_mask and implement db_vector_count()/db_vector_mask().
>>>
>>> - ntb_hw_epf:
>>>
>>>   - Document the legacy offset in ntb_epf_peer_db_set().
>>>   - Fix db_valid_mask to cover only real doorbell bits.
>>>   - Report 0-based db_vector to ntb_db_event() (accounting for the
>>>     unused slot).
>>>   - Pass per-vector context as request_irq() dev_id instead of deriving the
>>>     device vector from Linux IRQ numbers.
>>>   - Keep db_val as a bitmask and fix db_read/db_clear semantics
>>>     accordingly.
>>>   - Implement db_vector_count()/db_vector_mask().
>>>
>>>
>>> Compatibility
>>> =============
>>>
>>> By keeping the legacy offset intact, this series aims to remain
>>> compatible across mixed kernel versions. The observable changes are
>>> limited to correct mask/vector reporting and safer execution context
>>> handling.
>>>
>>> Patches 1-7 (PCI Endpoint) and 8-12 (NTB) are independent and can be
>>> applied separately through the respective trees. They are sent together so
>>> the two sides of the same legacy layout do not drift apart, and the
>>> resulting code stays easier to follow.
>>>
>>> The plan is still to take the whole series through the PCI EP tree once the
>>> remaining NTB acks are collected. See:
>>> https://lore.kernel.org/linux-pci/rnzsnp5de4qf5w7smebkmqekpuaqckltx73rj6ha3q2nrby5yp@7hsgvdzvjkp6/
>>>
>>>
>>> Note
>>> ====
>>>
>>> Most v3->v4 changes are responses to Sashiko feedback:
>>> https://sashiko.dev/#/patchset/20260323031544.2598111-1-den%40valinux.co.jp
>>>
>>> Sashiko also flagged a broader driver-unbind lifetime concern around the
>>> vNTB PCI driver. That is real, but it predates this series and needs a
>>> proper .remove() path, including NTB unregistration and work cancellation.
>>> I am keeping it as follow-up lifecycle work:
>>>
>>> https://lore.kernel.org/linux-pci/20260226084142.2226875-1-den@xxxxxxxxxxxxx/
>>>
>>> This v4 only clears the deferred doorbell state added by this series around
>>> EPC init/cleanup.
>>>
>>>
>>> ---
>>> Changelog
>>> =========
>>>
>>> Changes since v3:
>>>   - Rebased on the dependencies listed above.
>>>   - For Sashiko feedback on state added by this series: clear
>>>     peer_db_pending around work enable/disable, mask peer_db_set() with
>>>     db_valid_mask, and use __ffs64().
>>>   - For pre-existing issues made more relevant by per-vector semantics:
>>>     stop deriving ntb_hw_epf vectors from Linux IRQ numbers, pass
>>>     per-vector request_irq() context, ignore the reserved slot, and
>>>     validate vectors before BIT_ULL()/ntb_db_event().
>>>   - Added adjacent hardening: pci-epf-vntb db_count range/configfs guards,
>>>     bounded db_vector masks, ntb_hw_epf mw_count precheck, and helper guard
>>>     cleanups.
>>>   - Dropped Reviewed-by tags from patches that changed to non-trivial extent.
>>>
>>> Changes since v2:
>>>   - No functional changes.
>>>   - Rebased onto current pci/endpoint
>>>     e022f0c72c7f ("selftests: pci_endpoint: Skip reserved BARs").
>>>     * Patch 2 needed a trivial context-only adjustment while rebasing, due
>>>       to commit d799984233a5 ("PCI: endpoint: pci-epf-vntb: Stop
>>>       cmd_handler work in epf_ntb_epc_cleanup").
>>>   - Picked up additional Reviewed-by tags from Frank.
>>>   - Fixed the incorrect v2 series title.
>>>
>>> Changes since v1:
>>>   - Addressed feedback from Dave (add a source code comment, introduce
>>>     enum to eliminate magic numbers)
>>>   - Updated source code comment in Patch 2.
>>>   - No functional changes, so retained Reviewed-by tags by Frank and Dave.
>>>     Thank you both for the review.
>>>
>>> v3: https://lore.kernel.org/linux-pci/20260323031544.2598111-1-den@xxxxxxxxxxxxx/
>>> v2: https://lore.kernel.org/linux-pci/20260227084955.3184017-1-den@xxxxxxxxxxxxx/
>>> v1: https://lore.kernel.org/linux-pci/20260224133459.1741537-1-den@xxxxxxxxxxxxx/
>>>
>>>
>>> Best regards,
>>> Koichiro
>>>
>>>
>>> Koichiro Den (12):
>>>   PCI: endpoint: pci-epf-vntb: Document legacy MSI doorbell offset
>>>   PCI: endpoint: pci-epf-vntb: Defer pci_epc_raise_irq() out of atomic
>>>     context
>>>   PCI: endpoint: pci-epf-vntb: Report 0-based doorbell vector via
>>>     ntb_db_event()
>>>   PCI: endpoint: pci-epf-vntb: Reject unusable doorbell counts
>>>   PCI: endpoint: pci-epf-vntb: Guard configfs writes after EPC attach
>>>   PCI: endpoint: pci-epf-vntb: Exclude reserved slots from db_valid_mask
>>>   PCI: endpoint: pci-epf-vntb: Implement db_vector_count/mask for
>>>     doorbells
>>>   NTB: epf: Document legacy doorbell slot offset in
>>>     ntb_epf_peer_db_set()
>>>   NTB: epf: Make db_valid_mask cover only real doorbell bits
>>>   NTB: epf: Report 0-based doorbell vector via ntb_db_event()
>>>   NTB: epf: Fix doorbell bitmask and IRQ vector handling
>>>   NTB: epf: Implement db_vector_count/mask for doorbells
>>>
>>>  drivers/ntb/hw/epf/ntb_hw_epf.c               | 133 ++++++++---
>>>  drivers/pci/endpoint/functions/pci-epf-vntb.c | 210 ++++++++++++++++--
>>>  2 files changed, 296 insertions(+), 47 deletions(-)
>>>
>>
>