linux-kernel - [PATCH v3 11/12] x86/mtrr: don't let mtrr_type_lookup() return MTRR_TYPE

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Date:   Thu, 23 Feb 2023 10:32:42 +0100
From:   Juergen Gross <jgross@...e.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH v3 11/12] x86/mtrr: don't let mtrr_type_lookup() return MTRR_TYPE_INVALID

mtrr_type_lookup() should always return a valid memory type. In case
there is no information available, it should return the default UC.

This will remove the last case where mtrr_type_lookup() can return
MTRR_TYPE_INVALID, so adjust the comment in include/uapi/asm/mtrr.h.

Note that removing the MTRR_TYPE_INVALID #define from that header
could break user code, so it has to stay.

At the same time the mtrr_type_lookup() stub for the !CONFIG_MTRR
case should set uniform to 1, as if the memory range would be
covered by no MTRR at all.

Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Juergen Gross <jgross@...e.com>
---
V2:
- always set uniform
- set uniform to 1 in case of disabled MTRRs (Linus Torvalds)
V3:
- adjust include/uapi/asm/mtrr.h comment
---
 arch/x86/include/asm/mtrr.h        | 7 +++++--
 arch/x86/include/uapi/asm/mtrr.h   | 6 +++---
 arch/x86/kernel/cpu/mtrr/generic.c | 4 ++--
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
index f1cb81330a64..0d7ea8a54d81 100644
--- a/arch/x86/include/asm/mtrr.h
+++ b/arch/x86/include/asm/mtrr.h
@@ -59,9 +59,12 @@ static inline void mtrr_overwrite_state(struct mtrr_var_range *var,
 static inline u8 mtrr_type_lookup(u64 addr, u64 end, u8 *uniform)
 {
 	/*
-	 * Return no-MTRRs:
+	 * Return the default MTRR type, without any known other types in
+	 * that range.
 	 */
-	return MTRR_TYPE_INVALID;
+	*uniform = 1;
+
+	return MTRR_TYPE_UNCACHABLE;
 }
 #define mtrr_save_fixed_ranges(arg) do {} while (0)
 #define mtrr_save_state() do {} while (0)
diff --git a/arch/x86/include/uapi/asm/mtrr.h b/arch/x86/include/uapi/asm/mtrr.h
index 376563f2bac1..4aa05c2ffa78 100644
--- a/arch/x86/include/uapi/asm/mtrr.h
+++ b/arch/x86/include/uapi/asm/mtrr.h
@@ -115,9 +115,9 @@ struct mtrr_state_type {
 #define MTRR_NUM_TYPES       7
 
 /*
- * Invalid MTRR memory type.  mtrr_type_lookup() returns this value when
- * MTRRs are disabled.  Note, this value is allocated from the reserved
- * values (0x7-0xff) of the MTRR memory types.
+ * Invalid MTRR memory type.  No longer used outside of MTRR code.
+ * Note, this value is allocated from the reserved values (0x7-0xff) of
+ * the MTRR memory types.
  */
 #define MTRR_TYPE_INVALID    0xff
 
diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
index 9c9cbf6b56bc..af83fe6e2cb7 100644
--- a/arch/x86/kernel/cpu/mtrr/generic.c
+++ b/arch/x86/kernel/cpu/mtrr/generic.c
@@ -413,13 +413,13 @@ u8 mtrr_type_lookup(u64 start, u64 end, u8 *uniform)
 
 	if (!mtrr_state_set || !cache_map) {
 		*uniform = 0;	/* Uniformity is unknown. */
-		return MTRR_TYPE_INVALID;
+		return MTRR_TYPE_UNCACHABLE;
 	}
 
 	*uniform = 1;
 
 	if (!(mtrr_state.enabled & MTRR_STATE_MTRR_ENABLED))
-		return MTRR_TYPE_INVALID;
+		return MTRR_TYPE_UNCACHABLE;
 
 	for (i = 0; i < cache_map_n && start < end; i++) {
 		if (start >= cache_map[i].end)
-- 
2.35.3