[PATCH v2 2/2] KVM: Pre-allocate 1 cpumask variable per cpu for both pv tlb and pv ipis

[Wanpeng Li <kernellwp@xxxxxxxxx>]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

Nick Desaulniers Reported:

  When building with:
  $ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
  The following warning is observed:
  arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
  function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
  static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
  vector)
              ^
  Debugging with:
  https://github.com/ClangBuiltLinux/frame-larger-than
  via:
  $ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
    kvm_send_ipi_mask_allbutself
  points to the stack allocated `struct cpumask newmask` in
  `kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
  potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
  the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
  8192, making a single instance of a `struct cpumask` 1024 B.

This patch fixes it by pre-allocate 1 cpumask variable per cpu and use it for
both pv tlb and pv ipis..

Reported-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Acked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
v1 -> v2:
 * remove '!alloc' check
 * use new pv check helpers

 arch/x86/kernel/kvm.c | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 76ea8c4..377b224 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -432,6 +432,8 @@ static bool pv_tlb_flush_supported(void)
         kvm_para_has_feature(KVM_FEATURE_STEAL_TIME));
 }

+static DEFINE_PER_CPU(cpumask_var_t, __pv_cpu_mask);
+
 #ifdef CONFIG_SMP

 static bool pv_ipi_supported(void)
@@ -510,12 +512,12 @@ static void kvm_send_ipi_mask(const struct
cpumask *mask, int vector)
 static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask,
int vector)
 {
     unsigned int this_cpu = smp_processor_id();
-    struct cpumask new_mask;
+    struct cpumask *new_mask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
     const struct cpumask *local_mask;

-    cpumask_copy(&new_mask, mask);
-    cpumask_clear_cpu(this_cpu, &new_mask);
-    local_mask = &new_mask;
+    cpumask_copy(new_mask, mask);
+    cpumask_clear_cpu(this_cpu, new_mask);
+    local_mask = new_mask;
     __send_ipi_mask(local_mask, vector);
 }

@@ -595,7 +597,6 @@ static void __init kvm_apf_trap_init(void)
     update_intr_gate(X86_TRAP_PF, async_page_fault);
 }

-static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);

 static void kvm_flush_tlb_others(const struct cpumask *cpumask,
             const struct flush_tlb_info *info)
@@ -603,7 +604,7 @@ static void kvm_flush_tlb_others(const struct
cpumask *cpumask,
     u8 state;
     int cpu;
     struct kvm_steal_time *src;
-    struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_tlb_mask);
+    struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);

     cpumask_copy(flushmask, cpumask);
     /*
@@ -642,6 +643,7 @@ static void __init kvm_guest_init(void)
     if (pv_tlb_flush_supported()) {
         pv_ops.mmu.flush_tlb_others = kvm_flush_tlb_others;
         pv_ops.mmu.tlb_remove_table = tlb_remove_table;
+        pr_info("KVM setup pv remote TLB flush\n");
     }

     if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
@@ -748,24 +750,31 @@ static __init int activate_jump_labels(void)
 }
 arch_initcall(activate_jump_labels);

-static __init int kvm_setup_pv_tlb_flush(void)
+static __init int kvm_alloc_cpumask(void)
 {
     int cpu;
+    bool alloc = false;

     if (!kvm_para_available() || nopv)
         return 0;

-    if (pv_tlb_flush_supported()) {
+    if (pv_tlb_flush_supported())
+        alloc = true;
+
+#if defined(CONFIG_SMP)
+    if (pv_ipi_supported())
+        alloc = true;
+#endif
+
+    if (alloc)
         for_each_possible_cpu(cpu) {
-            zalloc_cpumask_var_node(per_cpu_ptr(&__pv_tlb_mask, cpu),
+            zalloc_cpumask_var_node(per_cpu_ptr(&__pv_cpu_mask, cpu),
                 GFP_KERNEL, cpu_to_node(cpu));
         }
-        pr_info("KVM setup pv remote TLB flush\n");
-    }

     return 0;
 }
-arch_initcall(kvm_setup_pv_tlb_flush);
+arch_initcall(kvm_alloc_cpumask);

 #ifdef CONFIG_PARAVIRT_SPINLOCKS

--
2.7.4