linux-kernel - Re: [PATCH] [next] KVM: lapic: allow setting apic debug dynamically

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Date:   Wed, 8 May 2019 14:59:31 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yi Wang <wang.yi59@....com.cn>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [next] KVM: lapic: allow setting apic debug dynamically

On Tue, May 07, 2019 at 02:37:33PM +0800, Yi Wang wrote:
> There are many functions invoke apic_debug(), which is defined
> as a null function by default, and that's incovenient for debuging
> lapic.
> 
> This patch allows setting apic debug according to add a apic_dbg
> parameter of kvm.
> 
> Signed-off-by: Yi Wang <wang.yi59@....com.cn>
> ---
>  arch/x86/kvm/lapic.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9bf70cf..4d8f10f 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -54,8 +54,13 @@
>  #define PRIu64 "u"
>  #define PRIo64 "o"
>  
> +static int apic_dbg;

s/int/bool to get this to compile.  And module params of this nature
should be tagged __read_mostly.

> +module_param(apic_dbg, bool, 0644);
> +
>  /* #define apic_debug(fmt,arg...) printk(KERN_WARNING fmt,##arg) */
> -#define apic_debug(fmt, arg...) do {} while (0)
> +#define apic_debug(fmt, arg...) do {  if (apic_dbg)   \
> +	printk(KERN_DEBUG fmt, ##arg);    \
> +} while (0)

I don't think we want to do this unless there is a very, very strong
argument to do so.  The reason debug messages like this are #ifdef'd
out is because they add conditional branches all over the place and
measurably increase the code size.  E.g.: this patch would increase
KVM's code footprint by 1100+ bytes:

 [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 2] .text             PROGBITS        0000000000000000 000070 04c282 00  AX  0   0 16

vs.

  [ 2] .text             PROGBITS        0000000000000000 000070 04c6d2 00  AX  0   0 16


>  
>  /* 14 is the version for Xeon and Pentium 8.4.8*/
>  #define APIC_VERSION			(0x14UL | ((KVM_APIC_LVT_NUM - 1) << 16))
> -- 
> 1.8.3.1
>