[tip: x86/mm] x86/kvm: Disable preemption in kvm_flush_tlb_multi()

[tip-bot2 for Chuyi Zhou]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     ae397bb32d2a4cb97d93539a5e50d1d081a7569a
Gitweb:        https://git.kernel.org/tip/ae397bb32d2a4cb97d93539a5e50d1d081a7569a
Author:        Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
AuthorDate:    Thu, 09 Jul 2026 20:29:32 +08:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Thu, 23 Jul 2026 12:30:31 +02:00

x86/kvm: Disable preemption in kvm_flush_tlb_multi()

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().

The x86/mm callers currently enter flush_tlb_multi() while pinned to a
CPU. To let those callers drop CPU pinning before issuing the remote TLB
flush, each PV backend must protect its own CPU-local scratch state.

Make the KVM backend protect its 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>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260709122933.4021501-14-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 dcef84d..5b554e4 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -663,8 +663,10 @@ 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