Merge patch series "riscv: Apply Zawrs when available"

Andrew Jones <ajones@ventanamicro.com> says:

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

Link: https://lore.kernel.org/r/20240426100820.14762-8-ajones@ventanamicro.com

* b4-shazam-merge:
  KVM: riscv: selftests: Add Zawrs extension to get-reg-list test
  KVM: riscv: Support guest wrs.nto
  riscv: hwprobe: export Zawrs ISA extension
  riscv: Add Zawrs support for spinlocks
  dt-bindings: riscv: Add Zawrs ISA extension description
  riscv: Provide a definition for 'pause'

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

1 files changed, 13 insertions, 7 deletions

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 3091d9539153..a96a8dadf662 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -600,6 +600,19 @@ config RISCV_ISA_V_PREEMPTIVE
 	  preemption. Enabling this config will result in higher memory
 	  consumption due to the allocation of per-task's kernel Vector context.
 
+config RISCV_ISA_ZAWRS
+	bool "Zawrs extension support for more efficient busy waiting"
+	depends on RISCV_ALTERNATIVE
+	default y
+	help
+	  The Zawrs extension defines instructions to be used in polling loops
+	  which allow a hart to enter a low-power state or to trap to the
+	  hypervisor while waiting on a store to a memory location. Enable the
+	  use of these instructions in the kernel when the Zawrs extension is
+	  detected at boot.
+
+	  If you don't know what to do here, say Y.
+
 config TOOLCHAIN_HAS_ZBB
 	bool
 	default y
@@ -682,13 +695,6 @@ config RISCV_ISA_ZICBOZ
 
 	   If you don't know what to do here, say Y.
 
-config TOOLCHAIN_HAS_ZIHINTPAUSE
-	bool
-	default y
-	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zihintpause)
-	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zihintpause)
-	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23600
-
 config TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI
 	def_bool y
 	# https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=aed44286efa8ae8717a77d94b51ac3614e2ca6dc