[PATCH 2/4] KVM: arm64: Don't mask softirq with IRQs disabled in vcpu_put()

[Dave Martin <Dave.Martin@xxxxxxx>]

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.

It is probably possible in theory to relax this restriction on
local_bh_disable()/_enable() usage, but for now this patch takes
the simple approach of managing softirq masking only if IRQs happen
to be enabled when kvm_arch_vcpu_put_fp() is called.

Fixes: e6b673b741ea ("KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing")
Reported-by: Andre Przywara <andre.przywara@xxxxxxx>
Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
---
 arch/arm64/kvm/fpsimd.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index dc6ecfa..b51ff80 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -90,10 +90,8 @@ void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
  * disappears and another task or vcpu appears that recycles the same
  * struct fpsimd_state.
  */
-void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
+static void __kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 {
-	local_bh_disable();
-
 	update_thread_flag(TIF_SVE,
 			   vcpu->arch.flags & KVM_ARM64_HOST_SVE_IN_USE);
 
@@ -105,6 +103,16 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 		/* Ensure user trap controls are correctly restored */
 		fpsimd_bind_task_to_cpu();
 	}
+}
+
 
-	local_bh_enable();
+void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
+{
+	if (irqs_disabled())
+		__kvm_arch_vcpu_put_fp(vcpu);
+	else {
+		local_bh_disable();
+		__kvm_arch_vcpu_put_fp(vcpu);
+		local_bh_enable();
+	}
 }
-- 
2.1.4

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