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Date:   Mon, 17 Feb 2020 21:15:48 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] KVM: Pre-allocate 1 cpumask variable per cpu for
 both pv tlb and pv ipis

On Mon, 17 Feb 2020 at 20:46, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Wanpeng Li <kernellwp@...il.com> writes:
>
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > 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@...gle.com>
> > Acked-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> > 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"

There is a lot of native ops we replace by pv ops in kvm.c, I use "KVM
setup xxx" there, like pv ipis, pv tlb flush, pv sched yield, should
we keep consistent as before?

    Wanpeng

>
> >      }
> >
> >      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@...hat.com>
>
> --
> Vitaly
>

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