Re: [PATCH 0/5] riscv: Apply Zawrs when available

[Andrew Jones <ajones@ventanamicro.com>]

On Fri, Mar 15, 2024 at 02:40:10PM +0100, 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.
> 
> 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.
> Currently, it's not expected that M-mode will configure and handle
> exceptions for timeouts (so it's expected that mstatus.TW=0), but 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 riscv smp_cond_load*
> functions which apply wrs.sto when available. We then switch from
> wrs.sto to wrs.nto, add hwprobe support (since the instructions are also
> usable from usermode), and finally teach KVM 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 based on kvm/queue and 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-v1/
> [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/2e712b19b7bb78634199bf262e6a75e09e1c87d2
> 
> Thanks,
> drew
> 
> 
> Andrew Jones (4):
>   riscv: Prefer wrs.nto over wrs.sto
>   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

I see I also forgot to add the patch for the entry in
Documentation/devicetree/bindings/riscv/extensions.yaml

Will do for v2.

Thanks,
drew

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