Re: [PATCH v6 00/16] x86/mtrr: fix handling with PAT but without MTRR

[Juergen Gross <jgross@suse.com>]

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On 11.05.23 18:32, Borislav Petkov wrote:
> On Wed, May 10, 2023 at 05:53:15PM +0200, Juergen Gross wrote:
>> Urgh, yes, there is something missing:
>>
>> diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
>> index 031f7ea8e72b..9544e7d13bb3 100644
>> --- a/arch/x86/kernel/cpu/mtrr/generic.c
>> +++ b/arch/x86/kernel/cpu/mtrr/generic.c
>> @@ -521,8 +521,12 @@ u8 mtrr_type_lookup(u64 start, u64 end, u8 *uniform)
>>          for (i = 0; i < cache_map_n && start < end; i++) {
>>                  if (start >= cache_map[i].end)
>>                          continue;
> 
> So the loop will go through the map until...
> 
>> -               if (start < cache_map[i].start)
>> +               if (start < cache_map[i].start) {
> 
> ... it reaches the first entry where that is true.
> 
>>                          type = type_merge(type, mtrr_state.def_type, uniform);
> 
> the @type argument is MTRR_TYPE_INVALID, def_type is WRBACK so what
> this'll do is simply get you the default WRBACK type:
> 
> type_merge:
>          if (type == MTRR_TYPE_INVALID)
>                  return new_type;
> 
>> +                       start = cache_map[i].start;
>> +                       if (end <= start)
>> +                               break;
> 
> Now you break here because end <= start. Why?
> 
> You can just as well do:
> 
> 	if (start < cache_map[i].start) {
> 		/* region non-overlapping with the region in the map */
> 		if (end <= cache_map[i].start)
> 			return type_merge(type, mtrr_state.def_type, uniform);
> 
> 		... rest of the processing ...
> 
> In general, I get it that your code is slick but I want it to be
> maintainable - not slick. I'd like for when people look at this, not
> have to  add a bunch of debugging output in order to swap the whole
> thing back into their brains.
> 
> So mtrr_type_lookup() definitely needs comments explaining what goes
> where.
> 
> You can send it as a diff ontop - I'll merge it.

The attached diff is for patch 13.


Juergen


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diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
index 031f7ea8e72b..4171788b8754 100644
--- a/arch/x86/kernel/cpu/mtrr/generic.c
+++ b/arch/x86/kernel/cpu/mtrr/generic.c
@@ -519,15 +519,26 @@ u8 mtrr_type_lookup(u64 start, u64 end, u8 *uniform)
 		return MTRR_TYPE_INVALID;
 
 	for (i = 0; i < cache_map_n && start < end; i++) {
+		/* Region after current map entry? -> continue with next one. */
 		if (start >= cache_map[i].end)
 			continue;
-		if (start < cache_map[i].start)
+
+		/* Start of region not covered by current map entry? */
+		if (start < cache_map[i].start) {
+			/* At least some part of region has default type. */
 			type = type_merge(type, mtrr_state.def_type, uniform);
+			/* End of region not covered, too? -> lookup done. */
+			if (end <= cache_map[i].start)
+				return type;
+		}
+
+		/* At least part of region covered by map entry. */
 		type = type_merge(type, cache_map[i].type, uniform);
 
 		start = cache_map[i].end;
 	}
 
+	/* End of region past last entry in map? -> use default type. */
 	if (start < end)
 		type = type_merge(type, mtrr_state.def_type, uniform);
 

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