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SubjectRe: [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@zte.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
>

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