From b045e4d0f392cbdab2674b0aa78c8d2b187e4e27 Mon Sep 17 00:00:00 2001 From: Dave Martin Date: Fri, 15 Jun 2018 16:47:24 +0100 Subject: [PATCH] KVM: arm64: Don't mask softirq with IRQs disabled in vcpu_put() Commit e6b673b ("KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing") introduces a specific helper kvm_arch_vcpu_put_fp() for saving the vcpu FPSIMD state during vcpu_put(). This function uses local_bh_disable()/_enable() to protect the FPSIMD context manipulation from interruption by softirqs. This approach is not correct, because vcpu_put() can be invoked either from the KVM host vcpu thread (when exiting the vcpu run loop), or via a preempt notifier. In the former case, only preemption is disabled. In the latter case, the function is called from inside __schedule(), which means that IRQs are disabled. Use of local_bh_disable()/_enable() with IRQs disabled is considerd an error, resulting in lockdep splats while running VMs if lockdep is enabled. This patch disables IRQs instead of attempting to disable softirqs, avoiding the problem of calling local_bh_enable() with IRQs disabled in the __schedule() path. This creates an additional interrupt blackout during vcpu run loop exit, but this is the rare case and the blackout latency is still less than that of __schedule(). Fixes: e6b673b741ea ("KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing") Reported-by: Andre Przywara Signed-off-by: Dave Martin Signed-off-by: Marc Zyngier --- arch/arm64/kvm/fpsimd.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c index dc6ecfa5a2d2..f9d09318b8db 100644 --- a/arch/arm64/kvm/fpsimd.c +++ b/arch/arm64/kvm/fpsimd.c @@ -5,7 +5,7 @@ * Copyright 2018 Arm Limited * Author: Dave Martin */ -#include +#include #include #include #include @@ -92,7 +92,9 @@ void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu) */ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu) { - local_bh_disable(); + unsigned long flags; + + local_irq_save(flags); update_thread_flag(TIF_SVE, vcpu->arch.flags & KVM_ARM64_HOST_SVE_IN_USE); @@ -106,5 +108,5 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu) fpsimd_bind_task_to_cpu(); } - local_bh_enable(); + local_irq_restore(flags); } -- 2.30.2