linux-kernel - [PATCH 1/2] x86/mtrr: Return success vs. "failure" from guest_force_mtrr

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Message-ID: <20250201005048.657470-2-seanjc@google.com>
Date: Fri, 31 Jan 2025 16:50:47 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>, Dionna Glaze <dionnaglaze@...gle.com>, 
	Peter Gonda <pgonda@...gle.com>, "Jürgen Groß" <jgross@...e.com>, 
	Kirill Shutemov <kirill.shutemov@...ux.intel.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, 
	"H . Peter Anvin" <hpa@...or.com>, Binbin Wu <binbin.wu@...el.com>, 
	Tom Lendacky <thomas.lendacky@....com>
Subject: [PATCH 1/2] x86/mtrr: Return success vs. "failure" from guest_force_mtrr_state()

When *potentially* forcing MTRRs to a single memory type, return whether
or not MTRRs were indeed overridden so that the caller can take additional
action when necessary.  E.g. KVM-as-a-guest will use the information to
also force the PAT memtype for legacy devices to be WB.

Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/include/asm/mtrr.h        |  5 +++--
 arch/x86/kernel/cpu/mtrr/generic.c | 11 +++++++----
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
index c69e269937c5..598753189f19 100644
--- a/arch/x86/include/asm/mtrr.h
+++ b/arch/x86/include/asm/mtrr.h
@@ -58,7 +58,7 @@ struct mtrr_state_type {
  */
 # ifdef CONFIG_MTRR
 void mtrr_bp_init(void);
-void guest_force_mtrr_state(struct mtrr_var_range *var, unsigned int num_var,
+bool guest_force_mtrr_state(struct mtrr_var_range *var, unsigned int num_var,
 			    mtrr_type def_type);
 extern u8 mtrr_type_lookup(u64 addr, u64 end, u8 *uniform);
 extern void mtrr_save_fixed_ranges(void *);
@@ -75,10 +75,11 @@ void mtrr_disable(void);
 void mtrr_enable(void);
 void mtrr_generic_set_state(void);
 #  else
-static inline void guest_force_mtrr_state(struct mtrr_var_range *var,
+static inline bool guest_force_mtrr_state(struct mtrr_var_range *var,
 					  unsigned int num_var,
 					  mtrr_type def_type)
 {
+	return false;
 }
 
 static inline u8 mtrr_type_lookup(u64 addr, u64 end, u8 *uniform)
diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
index 2fdfda2b60e4..4fd704907dbc 100644
--- a/arch/x86/kernel/cpu/mtrr/generic.c
+++ b/arch/x86/kernel/cpu/mtrr/generic.c
@@ -435,19 +435,21 @@ void __init mtrr_copy_map(void)
  * @var: MTRR variable range array to use
  * @num_var: length of the @var array
  * @def_type: default caching type
+ *
+ * Returns %true if MTRRs were overridden, %false if they were not.
  */
-void guest_force_mtrr_state(struct mtrr_var_range *var, unsigned int num_var,
+bool guest_force_mtrr_state(struct mtrr_var_range *var, unsigned int num_var,
 			    mtrr_type def_type)
 {
 	unsigned int i;
 
 	/* Only allowed to be called once before mtrr_bp_init(). */
 	if (WARN_ON_ONCE(mtrr_state_set))
-		return;
+		return false;
 
 	/* Only allowed when running virtualized. */
 	if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
-		return;
+		return false;
 
 	/*
 	 * Only allowed for special virtualization cases:
@@ -460,7 +462,7 @@ void guest_force_mtrr_state(struct mtrr_var_range *var, unsigned int num_var,
 	    !hv_is_isolation_supported() &&
 	    !cpu_feature_enabled(X86_FEATURE_XENPV) &&
 	    !cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
-		return;
+		return false;
 
 	/* Disable MTRR in order to disable MTRR modifications. */
 	setup_clear_cpu_cap(X86_FEATURE_MTRR);
@@ -480,6 +482,7 @@ void guest_force_mtrr_state(struct mtrr_var_range *var, unsigned int num_var,
 	mtrr_state.enabled |= MTRR_STATE_MTRR_ENABLED;
 
 	mtrr_state_set = 1;
+	return true;
 }
 
 static u8 type_merge(u8 type, u8 new_type, u8 *uniform)
-- 
2.48.1.362.g079036d154-goog