[PATCH v7 13/14] x86/kvm: Disable preemption in kvm_flush_tlb_multi()

[Chuyi Zhou]

kvm_flush_tlb_multi() is installed as an x86 PV TLB flush backend, so
flush_tlb_multi() can reach it through pv_ops when running as a KVM
guest.

kvm_flush_tlb_multi() uses the per-CPU scratch cpumask __pv_cpu_mask.
That buffer must remain tied to the current CPU until the mask has been
copied, filtered, and consumed by native_flush_tlb_multi().  Today the
x86/mm callers enter flush_tlb_multi() while pinned to a CPU, but a
subsequent x86/mm change will drop that caller-side CPU pinning before
issuing the remote TLB flush so the caller can be preempted while waiting
for remote CPUs.

Make the KVM backend protect its own per-CPU scratch cpumask by disabling
preemption locally.  This is harmless with the current callers, where the
preemption disable is nested, and makes the KVM pv_ops dependency explicit
before changing the x86/mm call sites.

Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
---
 arch/x86/kernel/kvm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 29226d112029..d540f54f4d16 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -662,8 +662,10 @@ static void kvm_flush_tlb_multi(const struct cpumask *cpumask,
 	u8 state;
 	int cpu;
 	struct kvm_steal_time *src;
-	struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
+	struct cpumask *flushmask;
 
+	guard(preempt)();
+	flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
 	cpumask_copy(flushmask, cpumask);
 	/*
 	 * We have to call flush only on online vCPUs. And
-- 
2.20.1