Re: [Qemu-devel] [PATCH] memory: use 128 bit in info mtree

[Michael S. Tsirkin]

On Mon, Mar 13, 2017 at 11:02:01AM +0800, Peter Xu wrote:
> On Sun, Mar 12, 2017 at 09:12:43PM +0200, Michael S. Tsirkin wrote:
> > info mtree is doing 64 bit math to figure out
> > addresses from offsets, this does not work ncorrectly
> > incase of overflow.
> > 
> > Overflow usually indicates a guest bug, so this is unusual
> > but reporting correct addresses makes it easier to discover
> > what is going on.
> 
> A tiny issue would be that we will always dump 128 bits even if
> nothing went wrong.
 
This is not what my patch is doing. It prints 16 digits if number fits.

> IMHO That's slightly awkward. Not sure whether
> that will confuse people since they should be thinking why we need
> that on 64bit systems...
> 
> Do you like below one instead? It'll keep the old interface, but just
> warn user explicity when something wrong happens, and it's much easier
> and obvious imho (along with a tiny cleanup):
> 
> (the code is not tested even for compile)

We don't use 64 bit addresses I don't really understand why does
it matter that 64 bit math would overflow.
It could be a valid configuration.

64 bit math overflow is just a particular case of a general issue where
all of child region is not visible through a parent.
IMHO if you are trying to report visible windows do just that:
add (visible through parent: AAA-BBB and maybe not visible through
parent).


> ---------8<-----------
> diff --git a/memory.c b/memory.c
> index 284894b..64b0a60 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -2494,6 +2494,7 @@ static void mtree_print_mr(fprintf_function mon_printf, 
> void *f,
>      MemoryRegionListHead submr_print_queue;
>      const MemoryRegion *submr;
>      unsigned int i;
> +    hwaddr cur_start, cur_end;
> 
>      if (!mr) {
>          return;
> @@ -2503,6 +2504,18 @@ static void mtree_print_mr(fprintf_function 
> mon_printf, void *f,
>          mon_printf(f, MTREE_INDENT);
>      }
> 
> +    cur_start = base + mr->addr;
> +    cur_end = cur_start + MR_SIZE(mr->size);
> +
> +    /*
> +     * Try to detect overflow of memory ranges. This should never
> +     * happen normally. When it happens, we dump something to warn the
> +     * user who is observing this.
> +     */
> +    if (cur_start < base || cur_end < cur_start) {
> +        mon_printf(f, "[DETECTED OVERFLOW!] ");
> +    }
> +
>      if (mr->alias) {
>          MemoryRegionList *ml;
>          bool found = false;
> @@ -2522,8 +2535,7 @@ static void mtree_print_mr(fprintf_function mon_printf, 
> void *f,
>          mon_printf(f, TARGET_FMT_plx "-" TARGET_FMT_plx
>                     " (prio %d, %s): alias %s @%s " TARGET_FMT_plx
>                     "-" TARGET_FMT_plx "%s\n",
> -                   base + mr->addr,
> -                   base + mr->addr + MR_SIZE(mr->size),
> +                   cur_start, cur_end,
>                     mr->priority,
>                     memory_region_type((MemoryRegion *)mr),
>                     memory_region_name(mr),
> @@ -2534,8 +2546,7 @@ static void mtree_print_mr(fprintf_function mon_printf, 
> void *f,
>      } else {
>          mon_printf(f,
>                     TARGET_FMT_plx "-" TARGET_FMT_plx " (prio %d, %s): 
> %s%s\n",
> -                   base + mr->addr,
> -                   base + mr->addr + MR_SIZE(mr->size),
> +                   cur_start, cur_end,
>                     mr->priority,
>                     memory_region_type((MemoryRegion *)mr),
>                     memory_region_name(mr),
> @@ -2562,7 +2573,7 @@ static void mtree_print_mr(fprintf_function mon_printf, 
> void *f,
>      }
> 
>      QTAILQ_FOREACH(ml, &submr_print_queue, queue) {
> -        mtree_print_mr(mon_printf, f, ml->mr, level + 1, base + mr->addr,
> +        mtree_print_mr(mon_printf, f, ml->mr, level + 1, cur_start,
>                         alias_print_queue);
>      }
> --------->8-----------
> 
> Thanks,
> 
> -- peterx