On Wed, Jun 26, 2024 at 07:34:48AM GMT, Palmer Dabbelt wrote: > On Mon, 03 Jun 2024 05:36:42 PDT (-0700), ajones@xxxxxxxxxxxxxxxx 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@xxxxxxxxxxxxxxxx/ > > > [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 > > >