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

[Vitaly Kuznetsov]

Wanpeng Li <kernellwp@xxxxxxxxx> writes:

> 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");

Nit: to be consistent with __send_ipi_mask() the message should be
somthing like

"KVM: switch to using PV TLB flush"

>      }
>
>      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
>

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly