linux-kernel - [tip: x86/sev] x86/boot: Introduce helpers for MSR reads/writes

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Date:   Fri, 08 Apr 2022 09:09:15 -0000
From:   "tip-bot2 for Michael Roth" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Borislav Petkov <bp@...e.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/sev] x86/boot: Introduce helpers for MSR reads/writes

The following commit has been merged into the x86/sev branch of tip:

Commit-ID:     176db622573f028f85221873ea4577e096785315
Gitweb:        https://git.kernel.org/tip/176db622573f028f85221873ea4577e096785315
Author:        Michael Roth <michael.roth@....com>
AuthorDate:    Wed, 09 Feb 2022 12:09:59 -06:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Wed, 06 Apr 2022 12:59:17 +02:00

x86/boot: Introduce helpers for MSR reads/writes

The current set of helpers used throughout the run-time kernel have
dependencies on code/facilities outside of the boot kernel, so there
are a number of call-sites throughout the boot kernel where inline
assembly is used instead. More will be added with subsequent patches
that add support for SEV-SNP, so take the opportunity to provide a basic
set of helpers that can be used by the boot kernel to reduce reliance on
inline assembly.

Use boot_* prefix so that it's clear these are helpers specific to the
boot kernel to avoid any confusion with the various other MSR read/write
helpers.

  [ bp: Disambiguate parameter names and trim comment. ]

Suggested-by: Borislav Petkov <bp@...en8.de>
Signed-off-by: Michael Roth <michael.roth@....com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-6-brijesh.singh@amd.com
---
 arch/x86/boot/msr.h               | 26 ++++++++++++++++++++++++++
 arch/x86/include/asm/msr.h        | 11 +----------
 arch/x86/include/asm/shared/msr.h | 15 +++++++++++++++
 3 files changed, 42 insertions(+), 10 deletions(-)
 create mode 100644 arch/x86/boot/msr.h
 create mode 100644 arch/x86/include/asm/shared/msr.h

diff --git a/arch/x86/boot/msr.h b/arch/x86/boot/msr.h
new file mode 100644
index 0000000..aed66f7
--- /dev/null
+++ b/arch/x86/boot/msr.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Helpers/definitions related to MSR access.
+ */
+
+#ifndef BOOT_MSR_H
+#define BOOT_MSR_H
+
+#include <asm/shared/msr.h>
+
+/*
+ * The kernel proper already defines rdmsr()/wrmsr(), but they are not for the
+ * boot kernel since they rely on tracepoint/exception handling infrastructure
+ * that's not available here.
+ */
+static inline void boot_rdmsr(unsigned int reg, struct msr *m)
+{
+	asm volatile("rdmsr" : "=a" (m->l), "=d" (m->h) : "c" (reg));
+}
+
+static inline void boot_wrmsr(unsigned int reg, const struct msr *m)
+{
+	asm volatile("wrmsr" : : "c" (reg), "a"(m->l), "d" (m->h) : "memory");
+}
+
+#endif /* BOOT_MSR_H */
diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index d42e6c6..65ec196 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -10,16 +10,7 @@
 #include <asm/errno.h>
 #include <asm/cpumask.h>
 #include <uapi/asm/msr.h>
-
-struct msr {
-	union {
-		struct {
-			u32 l;
-			u32 h;
-		};
-		u64 q;
-	};
-};
+#include <asm/shared/msr.h>
 
 struct msr_info {
 	u32 msr_no;
diff --git a/arch/x86/include/asm/shared/msr.h b/arch/x86/include/asm/shared/msr.h
new file mode 100644
index 0000000..1e6ec10
--- /dev/null
+++ b/arch/x86/include/asm/shared/msr.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_SHARED_MSR_H
+#define _ASM_X86_SHARED_MSR_H
+
+struct msr {
+	union {
+		struct {
+			u32 l;
+			u32 h;
+		};
+		u64 q;
+	};
+};
+
+#endif /* _ASM_X86_SHARED_MSR_H */