Re: [Xen-devel] [PATCH v4 03/15] xenctx: Add -m (--multiple_pages) option to output larger stack

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 03/20/2014 01:19 AM, Don Slutz wrote:
> On 03/19/14 11:34, George Dunlap wrote:
> > On 03/18/2014 10:15 PM, Don Slutz wrote:
> > > From: Don Slutz <Don@xxxxxxxxxxxxxxx>
> > >
> > > Important: This is the stack size to display not the configured
> > > stack size.
> > >
> > > Using pictures (for a 3 page configured system):
> > >
> > >         +------------------+
> > >         |                  |
> > >         |                  |
> > >         |                  |
> > >         |                  |
> > >         |                  |
> > >         +------------------+
> > >         |                  |
> > >         |                  |
> > >         |                  |
> > >         |                  |
> > >         |                  |
> > >         +------------------+
> > >         |                  |
> > >         |                  |
> > >         |                  |
> > > SP --> |                  |
> > >         |                  |
> > >         +------------------+
> > Sorry, what is this a picture of?  I can't make any sense out of it.  
> > Shouldn't this have only one box, the next have two, and the bottom 
> > one have three?
> This is various pictures of a 3 page stack, and where the SP currently 
> is.  Each box is a page.  So here the "stack limit" is the end of 1 
> page.  In the sense of how much stack is used, you are right it is 1, 
> 2 and then 3.  This tracks with the value passed for "-m" (see next 
> line).
Oh, right: so you're saying that the developer running xenctx has to 
*guess* how many pages are currently in use?  In other words, this 
really means, "Display N stack pages", and if you guessed too high, 
you'll get some pages worth of garbage.
[snip]
> > >     int main(int argc, char **argv)
> > >   {
> > >       int ch;
> > >       int ret;
> > > -    static const char *sopts = "fs:hak:SC";
> > > +    static const char *sopts = "fs:hak:SCm:";
> > >       static const struct option lopts[] = {
> > >           {"stack-trace", 0, NULL, 'S'},
> > >           {"symbol-table", 1, NULL, 's'},
> > >           {"frame-pointers", 0, NULL, 'f'},
> > >           {"kernel-start", 1, NULL, 'k'},
> > > +        {"multiple-pages", 0, NULL, 'm'},
> > I think I would call the long option "kernel-stack-pages" or 
> > something like that.  "Multiple pages" doesn't really convey much 
> > meaning.  -m is probably a fine short option, but -n might be more 
> > memorable.
> The issue with "kernel-stack-pages" is that it leads to configured 
> kernel stack pages (which for the pictures above is 3).  3 is most 
> likely not the number to use here.
> A big part of this is that how a "kernel" knows where it is in the 
> stack can be simple like for a 2 page stack, 1 page is odd, 2nd page 
> is even.  (3 pages is most likely more complex, but fence page(s) may 
> help here.)
> Maybe stack-limit-in-pages is better?
Hmm -- "--display-stack-pages", and then say, "Display N pages from the 
stack pointer"?
 -George

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