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

[Andrew Jones <ajones@ventanamicro.com>]

On Wed, Jun 26, 2024 at 07:34:48AM GMT, Palmer Dabbelt wrote:
> On Mon, 03 Jun 2024 05:36:42 PDT (-0700), ajones@ventanamicro.com wrote:
> > 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?
> 
> Sorry I forgot about this one, Anup just brought it up in the patchwork
> meeting.  He's going to ack the KVM bits, then I'll pick it up.

Thanks Palmer!

drew


> 
> > 
> > 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
> > >