Hi,
On 1/6/25 12:01 PM, Jonathan Cameron wrote:
On Mon, 6 Jan 2025 16:30:43 +0000
Wathsala Vithanage <wathsala.vithanage@xxxxxxx> wrote:
Linux v6.13-rc1 added support for PCIe TPH and direct cache injection.
As already described in the patch set[1] that introduced this feature,
the cache injection in supported hardware allows optimal utilization of
platform resources for specific requests on the PCIe bus. However, the
patch set [1] implements the functionality for usage within the kernel.
But certain user space applications, especially those whose performance
is sensitive to the latency of inbound writes as seen by a CPU core, may
benefit from using this information (E.g., the DPDK cache stashing
feature discussed in RFC [2]). This RFC is an attempt to obtain the PCIe
steering tag information from the kernel to be used by user mode
applications. We understand that there is more than one way to provide
this information. Please review and suggest alternatives if necessary.
The first of the two patches introduced in this RFC attempts to overcome
the kernel-only limitation by providing an API to kernel subsystems to
hook up relevant _DSM methods to a GENL interface. User space
applications can invoke a _DSM hooked up to this interface via the
"acpi-event" GENL family socket, granted they have the minimum
capabilities and message formats demanded by the kernel subsystem that
hooked up the _DSM method. This feature is added by extending the
"acpi-event" GENL family that multicasts ACPI events to the user-space
applications such as acpid.
The second patch of this RFC hooks up the PCIe root-port TLP Processing
Hints (TPH) _DSM to the ACPI GENL interface. User space applications
like [2] can now request the kernel to execute the _DSM on their behalf
and return steering-tag information.
[1] lore.kernel.org/linux-pci/20241002165954.128085-1-wei.huang2@xxxxxxx
[2] inbox.dpdk.org/dev/20241021015246.304431-2-wathsala.vithanage@xxxxxxx
Hi Wathsala,
Superficially this feels like another potential interface that could be wrapped
up under appropriate fwctl. Jason, what do you think?
Mind you I'm not personally convinced that an interface that focuses on
exposing _DSM calls to userspace makes sense as opposed to subsystem specific
stuff.
Maybe consider associating the actual interface with the individual PCI functions
(which provides the first chunk of the message directly).
Right,
I think this was similar to a conversation we had internally, which was
basically to detect the PCIe extended capability and export a 'steering'
entry in sysfs on each PCIe device which can take a logical cpu/cache
value, translate those on write to the ACPI cpu/cache id's, make the
firmware call, then directly update the PCIe device's capability with
the result. This also leaves the door open for future
cpu/cache->steering tag translation methods to transparently replace the
_DSM call while leaving the userspace API the same.
Also, _DSM is just one form of firmware interface used for PCI supporting
system. Tying the userspace interface to that feels unwise. I can certainly
foresee a PSCI/SCMI or similar interface for this on ARM platforms
wrapped up in _DSM where ACPI is present but directly accessed when DT
is in use.
I'd also request that you break out what goes in ARG0,1,2 as that is all
stuff that the kernel is aware of and not all reviewers have access to the
ECN (I do though). In particular the fact there are ACPI UIDs may
need a more generic solution.
Jonathan
Wathsala Vithanage (2):
ACPI: Add support for invoking select _DSM methods from user space
PCI: Add generic netlink interface to TPH _DSM
drivers/acpi/Makefile | 3 +-
drivers/acpi/{event.c => acpi_genl.c} | 110 ++++++++++++++++++++++-
drivers/acpi/acpi_genl_dsm.c | 76 ++++++++++++++++
drivers/pci/tph.c | 121 ++++++++++++++++++++++++++
include/acpi/acpi_genl.h | 54 ++++++++++++
include/linux/acpi.h | 1 +
6 files changed, 360 insertions(+), 5 deletions(-)
rename drivers/acpi/{event.c => acpi_genl.c} (63%)
create mode 100644 drivers/acpi/acpi_genl_dsm.c
create mode 100644 include/acpi/acpi_genl.h
