On Mon, Nov 27, 2023 at 11:37 AM Conor Dooley conor.dooley@microchip.com wrote:
Hi,
On Fri, Nov 24, 2023 at 08:21:37AM +0100, Christoph Muellner wrote:
From: Christoph Müllner christoph.muellner@vrull.eu
The upcoming RISC-V Ssdtso specification introduces a bit in the senvcfg CSR to switch the memory consistency model at run-time from RVWMO to TSO (and back). The active consistency model can therefore be switched on a per-hart base and managed by the kernel on a per-process/thread base.
This patch implements basic Ssdtso support and adds a prctl API on top so that user-space processes can switch to a stronger memory consistency model (than the kernel was written for) at run-time.
I am not sure if other architectures support switching the memory consistency model at run-time, but designing the prctl API in an arch-independent way allows reusing it in the future.
The patchset also comes with a short documentation of the prctl API.
This series is based on the second draft of the Ssdtso specification which was published recently on an RVI list: https://lists.riscv.org/g/tech-arch-review/message/183 Note, that the Ssdtso specification is in development state (i.e., not frozen or even ratified) which is also the reason why I marked the series as RFC.
One aspect that is not covered in this patchset is virtualization. It is planned to add virtualization support in a later version. Hints/suggestions on how to implement this part are very much appreciated.
Christoph Müllner (5):
I know this is an RFC, but it could probably do with a bit more compile testing, as:
RISC-V: Add basic Ssdtso support
This patch doesn't build for rv64 allmodconfig
RISC-V: Expose Ssdtso via hwprobe API
This one seems to build fine
uapi: prctl: Add new prctl call to set/get the memory consistency model RISC-V: Implement prctl call to set/get the memory consistency model RISC-V: selftests: Add DTSO tests
These don't build for: rv32 defconfig rv64 allmodconfig rv64 nommu
Thanks for reporting this. You are absolutely right. In my defense, this patchset was compile-tested and got some limited run-time testing in QEMU. But after that, I wrote the documentation, which triggered a renaming of several function/macro names, and these changes did not see adequate testing. I am sorry for that.
I've already fixed the patches (addressing the issues you have reported, plus other small issues). To not distract the ongoing discussion, I will not send an updated patchset right now. In case you are interested, you can find the latest changes (rebased on upstream/master) here: https://github.com/cmuellner/linux/tree/ssdtso I've also extended my local compile-test script to include all mentioned configs.
In case you want to play a bit with these changes, you can also have a look at the QEMU patchset, which also got support for the prctl (which is not part of the published mailpatch): https://github.com/cmuellner/qemu/tree/ssdtso With these changes, you can run the kernel self-test binary in user-mode emulation.
BR Christoph