Re: git: cc21f6e53f43 - main - amd64: Make a comment less confusing

On 7/21/25 10:58, Mark Johnston wrote:
> The branch main has been updated by markj:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=cc21f6e53f43f0e4c082222460d90ea581f3e2d6
> 
> commit cc21f6e53f43f0e4c082222460d90ea581f3e2d6
> Author:     Mark Johnston <markj@FreeBSD.org>
> AuthorDate: 2025-07-21 13:33:31 +0000
> Commit:     Mark Johnston <markj@FreeBSD.org>
> CommitDate: 2025-07-21 14:57:58 +0000
> 
>      amd64: Make a comment less confusing
>      
>      Also merge two printf()s.  No functional change intended.
>      
>      Reviewed by:    kib
>      MFC after:      1 week
>      Differential Revision:  https://reviews.freebsd.org/D51452
> ---
>   sys/amd64/amd64/trap.c | 13 ++++++-------
>   1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/sys/amd64/amd64/trap.c b/sys/amd64/amd64/trap.c
> index d9a5f7096a6e..f3469ed5e2bc 100644
> --- a/sys/amd64/amd64/trap.c
> +++ b/sys/amd64/amd64/trap.c
> @@ -37,7 +37,6 @@
>    * SUCH DAMAGE.
>    */
>   
> -#include <sys/cdefs.h>
>   /*
>    * AMD64 Trap and System call handling
>    */
> @@ -898,9 +897,9 @@ trap_diag(struct trapframe *frame, vm_offset_t eva)
>   	printf("\n\nFatal trap %d: %s while in %s mode\n", type,
>   	    type < nitems(trap_msg) ? trap_msg[type] : UNKNOWN,
>   	    TRAPF_USERMODE(frame) ? "user" : "kernel");
> -	/* two separate prints in case of a trap on an unmapped page */
> -	printf("cpuid = %d; ", PCPU_GET(cpuid));
> -	printf("apic id = %02x\n", PCPU_GET(apic_id));
> +	/* Print these separately in case pcpu accesses trap. */
> +	printf("cpuid = %d; apic id = %02x\n", PCPU_GET(cpuid),
> +	    PCPU_GET(apic_id));
>   	if (type == T_PAGEFLT) {
>   		printf("fault virtual address	= 0x%lx\n", eva);
>   		printf("fault code		= %s %s %s%s%s, %s\n",
> @@ -1021,9 +1020,9 @@ dblfault_handler(struct trapframe *frame)
>   	    frame->tf_cs, frame->tf_ss, frame->tf_ds, frame->tf_es,
>   	    frame->tf_fs, frame->tf_gs,
>   	    rdmsr(MSR_FSBASE), rdmsr(MSR_GSBASE), rdmsr(MSR_KGSBASE));
> -	/* two separate prints in case of a trap on an unmapped page */
> -	printf("cpuid = %d; ", PCPU_GET(cpuid));
> -	printf("apic id = %02x\n", PCPU_GET(apic_id));
> +	/* Print these separately in case pcpu accesses trap. */
> +	printf("cpuid = %d; apic id = %02x\n", PCPU_GET(cpuid),
> +	    PCPU_GET(apic_id));
>   	panic("double fault");

Eh, if any of the accesses fault you don't get any of the printf, so once
you've merged these, the comment is wrong and should just be removed.

FWIW, pre-5.0 (maybe pre-4.0?) per-CPU data on i386 used a fixed virtual
address that was mapped to different backing pages on each CPU instead of
%gs which is why the comment was probably worded that way originally.

(This meant, btw, that each CPU had its own set of top-level kernel page
table and that cpu_switch had to munge the kernel-half of the top-level
page table of a process, which precluded multiple threads in a process.)

-- 
John Baldwin