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

From: Juergen Gross
Date: Mon May 22 2023 - 10:18:02 EST

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

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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