| Message ID | 20250106163045.508959-1-wathsala.vithanage@arm.com |
|---|---|
| Headers | show |
| Series | GENL interface for ACPI _DSM methods | expand |
On Tue, Jan 07, 2025 at 11:37:01AM -0600, Jeremy Linton wrote: > Hi, > > On 1/6/25 12:01 PM, Jonathan Cameron wrote: > > On Mon, 6 Jan 2025 16:30:43 +0000 > > Wathsala Vithanage <wathsala.vithanage@arm.com> 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]). There is no way for userspace to program TPH information into a PCI device without going through a kernel driver, and the kernel driver must be the exclusive owner of the steering tag configuration or chaos would ensue. Having a way for sysfs to override this seems very wrong to me, and I think you should not go in this direction. DPDK runs on VFIO or RDMA. It would natural to have an VFIO native API to manipulate the steering tags, and we are already discussing what RDMA support for steering tag would look like. > > Superficially this feels like another potential interface that could be wrapped > > up under appropriate fwctl. Jason, what do you think? As above, I think this very squarely belongs under the appropriate subsystems that are providing the kernel drivers for the device. There is no reasonable way to share steering tags with unrelated userspace through any mechanism. Basically it fails the independence test of fwctl. > 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. Seems wrong, driver must do this. If the driver was already using that entry for something else you've just wrecked it. Jason
On Wed, Jan 08, 2025 at 01:59:35PM -0600, Jeremy Linton wrote: > I'm under the impression this is a similar problem to cpu/irq/numa affinity > where the driver/subsystem should be making the choice, but the user is > provided the opportunity to override the defaults if they think there is > benefit in their environment. Which I think has been proven to have been a mistake. Instead over overriding irq affinity though proc/irq under the covers of the driver and hoping for the best the driver itself should have the opportinuty to set the affinity for its objects directly. Lets us not repeat this mistake with steering tag. The driver should always be involved in this stuff, if you want it to work with DPDK then go through the kernel driver that DPDK is running on top of (VFIO or RDMA) Jason
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@amd.com [2] inbox.dpdk.org/dev/20241021015246.304431-2-wathsala.vithanage@arm.com 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