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

On 7/21/25 12:43, Mark Johnston wrote:
> On Mon, Jul 21, 2025 at 12:37:23PM -0400, John Baldwin wrote:
>> 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,
> 
> Right, but I believe the comment is referring to the preceding printf,
> which doesn't print any PCPU fields.  In what situation would
> PCPU_GET(apic_id) fault while PCPU_GET(cpuid) succeeds?
> 
> (Yes, my commit message here should have been better.)

Oh, woof.  The second printf used to read from the lapic directly instead
of per-CPU data.  I changed it here:

https://svnweb.freebsd.org/base?view=revision&revision=121986

So prior to that each printf was indeed reading from separate pages:

#ifdef SMP
         /* two separate prints in case of a trap on an unmapped page */
         printf("cpuid = %d; ", PCPU_GET(cpuid));
         printf("lapic.id = %08x\n", lapic.id);
#endif

The comment used to reference _three_ printfs prior to the SMPng commit:

https://svnweb.freebsd.org/base/head/sys/i386/i386/trap.c?r1=65556&r2=65557&

The original comment (referencing 3) was added by peter@ here:

https://svnweb.freebsd.org/base?view=revision&revision=29128

It certainly seems to only apply to the printfs under #ifdef SMP.

Removing the comment has the additional property of fixing the style bug
of not having a blank line before the comment that was previously provided
by the #ifdef SMP. :)

-- 
John Baldwin