linux-kernel - [PATCH 0/3] LoongArch: KVM: Add paravirt preempt hint support

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Message-Id: <20251118080656.2012805-1-maobibo@loongson.cn>
Date: Tue, 18 Nov 2025 16:06:53 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Paolo Bonzini <pbonzini@...hat.com>,
	Huacai Chen <chenhuacai@...nel.org>
Cc: kvm@...r.kernel.org,
	loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH 0/3] LoongArch: KVM: Add paravirt preempt hint support

vCPU preempt hint is useful with sched and lock on some platforms, here
new feature KVM_FEATURE_PREEMPT_HINT is added and VMM can selectively
enable it.

Test case kcbench is used to compile Linux kernel code, the test result
shows that it is useful on 3D6000 Dual-way machine with 64 cores and 128
hyperthreads, however no improvemet on 3C5000 Dual-way machine with 32
cores. With perf top command when running test case, the main difference
between over-commited VM and host is osq_lock(), it can avoid
unnecessary busy-loop waiting and enter sleep state quickly if lock-hold
vCPU is preempted.

Here is test result with kcbench, time unit is second to compile kernel
with defconfig, performance is better with smaller value.
3D6000 Dual-way 64 Core 128 Threads
 One VM with 128 vCPUs, no overcommit, NUMA
             Orginal       With-patch       Improvement
  VM         91.72         92.4             < -1%
  Host       89.7          89.75            < -0.1%
 Two VMs overcommit with 128 vCPUs, UMA
             Orginal       With-patch       Improvement
  VM1        306.9         197.5            36%
  VM2        303.7         197.8            35%
  Host       89.7          89.75             < -0.1%
 Two VMs overcommit with 128 vCPUs, NUMA
             Orginal       With-patch       Improvement
  VM1        317.1         159              50%
  VM2        317.5         158              50%
  Host       89.7          89.75            < -0.1%
3C5000  Dual-way 32 Core
 One VM with 32 vCPUs, NUMA
             Orginal       With-patch       Improvement
  VM         208           207              < 0.5%
  Host       184           185              < -0.5%
 Two VMs overcommit with 32 vCPUs, UMA
             Orginal       With-patch       Improvement
  VM1        439           444              -1%
  VM2        437           438              < -0.2%
  Host       184           185              < -0.5%
 Two VMs overcommit with 32 vCPUs, NUMA
             Orginal       With-patch       Improvement
  VM1        422           425              < -1%
  VM2        418           415              < -1%
  Host       184           185              < -0.5%

Bibo Mao (3):
  LoongArch: KVM: Add preempt hint feature in hypervisor side
  LoongArch: Add paravirt support with vcpu_is_preempted()
  LoongArch: Add paravirt preempt hint print prompt

 arch/loongarch/include/asm/kvm_host.h      |  2 +
 arch/loongarch/include/asm/kvm_para.h      |  5 +-
 arch/loongarch/include/asm/smp.h           |  1 +
 arch/loongarch/include/asm/spinlock.h      |  5 ++
 arch/loongarch/include/uapi/asm/kvm.h      |  1 +
 arch/loongarch/include/uapi/asm/kvm_para.h |  1 +
 arch/loongarch/kernel/paravirt.c           | 24 +++++++++-
 arch/loongarch/kernel/smp.c                |  6 +++
 arch/loongarch/kvm/vcpu.c                  | 54 +++++++++++++++++++++-
 arch/loongarch/kvm/vm.c                    |  5 +-
 10 files changed, 100 insertions(+), 4 deletions(-)


base-commit: 6a23ae0a96a600d1d12557add110e0bb6e32730c
-- 
2.39.3