Re: [Xen-devel] [PATCH] xen/arm: add warning if memory modules overlap

[Julien Grall <julien.grall@arm.com>]

Hi,

On 10/11/19 7:06 PM, Brian Woods wrote:
> On Fri, Oct 11, 2019 at 05:58:35PM +0100, Julien Grall wrote:
> For that, you'd need to either check the start and end of the added
> module or the start of both.  So something like:
> 
> if ( ((mod->start >= start) && (mod->start < (start + size))) ||
>       ((start >= mod->start) && (start < (mod->start + mod->size))) )
>      printk("WARNING: ...");
> 
> If you don't you run the risk of having a module overlap but not at the
> start of the added module (unless there's a guaranteed order).  You're
> still running all of the checks from what I can tell, just not in nested
> for loop so. Plus I'm not sure how many times add_boot_module gets run
> and the "mod->kind == kind .." if statement gets run and is true.
> If the above is true, wouldn't that cause extra checks for the for loop
> iterations before it was true?

For non-dom0less case, we are talking about 4 modules max (Xen, Kernel, 
Initramfs, flask policy). Modules cannot be the shared here.

For dom0less, you are unlikely to have that many domains started from 
Xen. So the number of modules will still be limited (even more if you 
share it).

This code is also only called at boot where there are bigger time 
consuming part (such as domheap initialization). So I would be surprised 
if you see any improvement (other than a couple of cycles) in boot time 
here.

Therefore, I would favor a readable solution over a micro-optimized 
solution here.

Cheers,

-- 
Julien Grall

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