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

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On 31.05.23 19:48, Borislav Petkov wrote:
On Wed, May 31, 2023 at 04:20:08PM +0200, Juergen Gross wrote:
One other note: why does mtrr_cleanup() think that using 8 instead of 6
variable MTRRs would be an "optimal setting"?

Maybe the more extensive debug output below would help answer that...

IMO it should replace the original setup only in case it is using _less_
MTRRs than before.

Right.

The attached patch will do that.


Juergen

From 7989ef9822115a708fc2ba3f7740888a350cb40f Mon Sep 17 00:00:00 2001
From: Juergen Gross <jgross@xxxxxxxx>
Date: Thu, 1 Jun 2023 14:40:58 +0200
Subject: [PATCH v7] x86/mtrr: Let mtrr_cleanup() not increase number of used
 MTRRs

Today mtrr_cleanup() will always use the best found alternative MTRR
setting, even if this setting is using more variable MTRRs than the
BIOS provided setup.

Add a check that only settings with less variable MTRRs are used.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V7:
- new patch
---
 arch/x86/kernel/cpu/mtrr/cleanup.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/mtrr/cleanup.c b/arch/x86/kernel/cpu/mtrr/cleanup.c
index a7cb5d32d03d..a5d331722092 100644
--- a/arch/x86/kernel/cpu/mtrr/cleanup.c
+++ b/arch/x86/kernel/cpu/mtrr/cleanup.c
@@ -567,7 +567,7 @@ static int __init mtrr_need_cleanup(void)
 	    num_var_ranges - num[MTRR_NUM_TYPES])
 		return 0;
 
-	return 1;
+	return num_var_ranges - num[MTRR_NUM_TYPES];
 }
 
 static unsigned long __initdata range_sums;
@@ -673,6 +673,7 @@ int __init mtrr_cleanup(void)
 	u64 chunk_size, gran_size;
 	mtrr_type type;
 	int index_good;
+	int num_used;
 	int i;
 
 	if (!cpu_feature_enabled(X86_FEATURE_MTRR) || enable_mtrr_cleanup < 1)
@@ -693,7 +694,8 @@ int __init mtrr_cleanup(void)
 	}
 
 	/* Check if we need handle it and can handle it: */
-	if (!mtrr_need_cleanup())
+	num_used = mtrr_need_cleanup();
+	if (!num_used)
 		return 0;
 
 	/* Print original var MTRRs at first, for debugging: */
@@ -728,6 +730,10 @@ int __init mtrr_cleanup(void)
 		mtrr_print_out_one_result(i);
 
 		if (!result[i].bad) {
+			if (result[i].num_reg >= num_used) {
+				Dprintk("BIOS provided MTRR setting is better than found one\n");
+				return 0;
+			}
 			set_var_mtrr_all();
 			Dprintk("New variable MTRRs\n");
 			print_out_mtrr_range_state();
@@ -762,8 +768,12 @@ int __init mtrr_cleanup(void)
 	index_good = mtrr_search_optimal_index();
 
 	if (index_good != -1) {
-		pr_info("Found optimal setting for mtrr clean up\n");
 		i = index_good;
+		if (result[i].num_reg >= num_used) {
+			Dprintk("BIOS provided MTRR setting is better than found one\n");
+			return 0;
+		}
+		pr_info("Found optimal setting for mtrr clean up\n");
 		mtrr_print_out_one_result(i);
 
 		/* Convert ranges to var ranges state: */
-- 
2.35.3

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