[PATCH v3 0/6] riscv: Apply Zawrs when available

[Andrew Jones]

Hi Palmer,

I submit our concerns of wrs.nto to RVI ARC for consideration. They
discussed it but don't believe there's a need for concern. The
expectation is that there will always be enough interrupt activity
and that those interrupts with activity will not likely be locally
disabled.

Can we consider this series for 6.10?

Thanks,
drew

On Fri, Apr 26, 2024 at 12:08:20PM GMT, Andrew Jones wrote:
> Zawrs provides two instructions (wrs.nto and wrs.sto), where both are
> meant to allow the hart to enter a low-power state while waiting on a
> store to a memory location. The instructions also both wait an
> implementation-defined "short" duration (unless the implementation
> terminates the stall for another reason). The difference is that while
> wrs.sto will terminate when the duration elapses, wrs.nto, depending on
> configuration, will either just keep waiting or an ILL exception will be
> raised. Linux will use wrs.nto, so if platforms have an implementation
> which falls in the "just keep waiting" category (which is not expected),
> then it should _not_ advertise Zawrs in the hardware description.
> 
> Like wfi (and with the same {m,h}status bits to configure it), when
> wrs.nto is configured to raise exceptions it's expected that the higher
> privilege level will see the instruction was a wait instruction, do
> something, and then resume execution following the instruction. For
> example, KVM does configure exceptions for wfi (hstatus.VTW=1) and
> therefore also for wrs.nto. KVM does this for wfi since it's better to
> allow other tasks to be scheduled while a VCPU waits for an interrupt.
> For waits such as those where wrs.nto/sto would be used, which are
> typically locks, it is also a good idea for KVM to be involved, as it
> can attempt to schedule the lock holding VCPU.
> 
> This series starts with Christoph's addition of the riscv
> smp_cond_load_relaxed function which applies wrs.sto when available.
> That patch has been reworked to use wrs.nto and to use the same approach
> as Arm for the wait loop, since we can't have arbitrary C code between
> the load-reserved and the wrs. Then, hwprobe support is added (since the
> instructions are also usable from usermode), and finally KVM is
> taught about wrs.nto, allowing guests to see and use the Zawrs
> extension.
> 
> We still don't have test results from hardware, and it's not possible to
> prove that using Zawrs is a win when testing on QEMU, not even when
> oversubscribing VCPUs to guests. However, it is possible to use KVM
> selftests to force a scenario where we can prove Zawrs does its job and
> does it well. [4] is a test which does this and, on my machine, without
> Zawrs it takes 16 seconds to complete and with Zawrs it takes 0.25
> seconds.
> 
> This series is also available here [1]. In order to use QEMU for testing
> a build with [2] is needed. In order to enable guests to use Zawrs with
> KVM using kvmtool, the branch at [3] may be used.
> 
> [1] https://github.com/jones-drew/linux/commits/riscv/zawrs-v3/
> [2] https://lore.kernel.org/all/20240312152901.512001-2-ajones@ventanamicro.com/
> [3] https://github.com/jones-drew/kvmtool/commits/riscv/zawrs/
> [4] https://github.com/jones-drew/linux/commit/cb2beccebcece10881db842ed69bdd5715cfab5d
> 
> Thanks,
> drew
> 
> v3:
>  - Moved comment about expected termination from the DT binding text
>    to a code comment.
> 
> v2:
>  - Added DT bindings patch with additional Linux specifications due
>    to wrs.nto potentially never terminating, as suggested by Palmer
>  - Added patch to share pause insn definition
>  - Rework main Zawrs support patch to use Arm approach (which is
>    also the approach that Andrea Parri suggested)
>  - Dropped the riscv implementation of smp_cond_load_acquire().
>    afaict, the generic implementation, which will use the riscv
>    implementation of smp_cond_load_relaxed() is sufficient for riscv.
>  - The rework was large enough (IMO) to drop Heiko's s-o-b and to
>    add myself as a co-developer
> 
> 
> Andrew Jones (5):
>   riscv: Provide a definition for 'pause'
>   dt-bindings: riscv: Add Zawrs ISA extension description
>   riscv: hwprobe: export Zawrs ISA extension
>   KVM: riscv: Support guest wrs.nto
>   KVM: riscv: selftests: Add Zawrs extension to get-reg-list test
> 
> Christoph M??llner (1):
>   riscv: Add Zawrs support for spinlocks
> 
>  Documentation/arch/riscv/hwprobe.rst          |  4 ++
>  .../devicetree/bindings/riscv/extensions.yaml |  7 +++
>  arch/riscv/Kconfig                            | 20 ++++---
>  arch/riscv/Makefile                           |  3 -
>  arch/riscv/include/asm/barrier.h              | 45 +++++++++-----
>  arch/riscv/include/asm/cmpxchg.h              | 58 +++++++++++++++++++
>  arch/riscv/include/asm/hwcap.h                |  1 +
>  arch/riscv/include/asm/insn-def.h             |  4 ++
>  arch/riscv/include/asm/kvm_host.h             |  1 +
>  arch/riscv/include/asm/vdso/processor.h       |  8 +--
>  arch/riscv/include/uapi/asm/hwprobe.h         |  1 +
>  arch/riscv/include/uapi/asm/kvm.h             |  1 +
>  arch/riscv/kernel/cpufeature.c                |  1 +
>  arch/riscv/kernel/sys_hwprobe.c               |  1 +
>  arch/riscv/kvm/vcpu.c                         |  1 +
>  arch/riscv/kvm/vcpu_insn.c                    | 15 +++++
>  arch/riscv/kvm/vcpu_onereg.c                  |  2 +
>  .../selftests/kvm/riscv/get-reg-list.c        |  4 ++
>  18 files changed, 146 insertions(+), 31 deletions(-)
> 
> -- 
> 2.44.0
>