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

Juergen Gross
2023-05-22T14:18:05Z

[-- Attachment #1.1.1: Type: text/plain, Size: 2068 bytes --]

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

[-- Attachment #1.1.2: diff.patch --]
[-- Type: text/x-patch, Size: 1143 bytes --]

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);

[-- Attachment #1.1.3: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3149 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]