Re: [PATCH v8 03/20] xenctx: Add -n (--display-stack-pages) option to output larger stack

[George Dunlap <george.dunlap@eu.citrix.com>]

On 04/01/2014 02:19 PM, Ian Campbell wrote:
> On Thu, 2014-03-27 at 15:05 -0400, Don Slutz wrote:
>> Important: This is the stack size (also known as stack limit) to
>> display not the configured stack size.
>>
>> Note: use with caution (easy to get garbage).
>>
>> Below is a pictures of a configured 3 page stack, and where
>> the SP currently is.  Each box is a page.
>>
>>                              -n 1    -n 2    -n 3
>>
>>         +------------------+
>>         |                  |
>>         |                  |
>>         |                  |
>>         |                  |
>> SP --> |                  |   *       *       *
>>         +------------------+   	      |	      |
>>         |                  |	      |	      |
>>         |                  |	      |	      |
>>         |                  |	      |	      |
>>         |                  |	      |	      |
>>         |                  |	      *	      |
>>         +------------------+	      	      |
>>         |                  |		      |
>>         |                  |		      |
>>         |                  |		      |
>>         |                  |		      |
>>         |                  |		      *
>>         +------------------+
>>
>> Display using "-n 3" since the used stack pages is 3.
>
> Stacks grow downwards on all of the architectures we support. Perhaps
> that is why the rest of us find your diagrams so confusing?
>
> I drew my example to you the way I did for a reason.
>
> Also, should the other end of the *---* line from SP (the bottom in your
> diagram above) not be aligned to a page boundary, after all -n works in
> pages.
>
>
>> @@ -664,6 +667,8 @@ static int print_stack(vcpu_guest_context_any_t *ctx, int vcpu, int width)
>>
>>       stack_limit = ((stack_pointer(ctx) + XC_PAGE_SIZE)
>>                      & ~((guest_word_t) XC_PAGE_SIZE - 1));
>> +    if ( xenctx.nr_stack_pages > 1 )
>> +        stack_limit += (xenctx.nr_stack_pages - 1) * XC_PAGE_SIZE;
>
> The if here is still redundant.
>
>>       printf("\n");
>>       printf("Stack:\n");
>>       for (i=1; i<5 && stack < stack_limit; i++) {
>> @@ -834,18 +839,24 @@ static void usage(void)
>>           kernel_start);
>>       printf("  -a, --all          display more registers\n");
>>       printf("  -C, --all-vcpus    print info for all vcpus\n");
>> +    printf("  -n PAGES, --display-stack-pages=PAGES\n");
>> +    printf("                     Display N pages from the stack pointer. (default %d)\n",
>> +           DEFAULT_NR_STACK_PAGES);
>> +    printf("                     Changes stack limit.  Note: use with caution (easy\n");
>> +    printf("                     to get garbage).\n");
>
> Doesn't it go without saying that if you go off the bottom of the stack
> you will get garbage?

Only if you know that the tool doesn't know where the end of the stack 
is.  (Although hopefully the wording, "Display N pages" should give you 
a hint.)

But what is "changes stack limit" supposed to mean?

  -George