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Date:   Mon, 27 Jan 2020 09:09:36 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nick Desaulniers <nick.desaulniers@gmail.com>
Cc:     pbonzini@redhat.com, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
Subject: Re: [PATCH] dynamically allocate struct cpumask
Message-ID: <20200127080935.GH14914@hirez.programming.kicks-ass.net>
References: <20200127071602.11460-1-nick.desaulniers@gmail.com>
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On Sun, Jan 26, 2020 at 11:16:02PM -0800, Nick Desaulniers wrote:
> This helps avoid avoid a potentially large stack allocation.
> 
> 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.
> 
> Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
> ---
>  arch/x86/kernel/kvm.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 32ef1ee733b7..d41c0a0d62a2 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -494,13 +494,15 @@ 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;

Right, on stack cpumask is definitely dodgy.

> +	struct cpumask *new_mask;
>  	const struct cpumask *local_mask;
>  
> -	cpumask_copy(&new_mask, mask);
> -	cpumask_clear_cpu(this_cpu, &new_mask);
> -	local_mask = &new_mask;
> +	new_mask = kmalloc(sizeof(*new_mask), GFP_KERNEL);
> +	cpumask_copy(new_mask, mask);
> +	cpumask_clear_cpu(this_cpu, new_mask);
> +	local_mask = new_mask;
>  	__send_ipi_mask(local_mask, vector);
> +	kfree(new_mask);
>  }

One alternative approach is adding the inverse of cpu_bit_bitmap. I'm
not entirely sure how often we need the all-but-self mask, but ISTR
there were other places too.