linux-kernel - [PATCH v10 05/45] x86/boot: Introduce helpers for MSR reads/writes

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Date:   Wed, 9 Feb 2022 12:09:59 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>, <linux-efi@...r.kernel.org>,
        <platform-driver-x86@...r.kernel.org>,
        <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "Vitaly Kuznetsov" <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        <brijesh.ksingh@...il.com>, <tony.luck@...el.com>,
        <marcorr@...gle.com>, <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: [PATCH v10 05/45] x86/boot: Introduce helpers for MSR reads/writes

From: Michael Roth <michael.roth@....com>

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.

Suggested-by: Borislav Petkov <bp@...en8.de>
Signed-off-by: Michael Roth <michael.roth@....com>
---
 arch/x86/boot/msr.h               | 28 ++++++++++++++++++++++++++++
 arch/x86/include/asm/msr.h        | 11 +----------
 arch/x86/include/asm/shared/msr.h | 15 +++++++++++++++
 3 files changed, 44 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 000000000000..b6bb2161da27
--- /dev/null
+++ b/arch/x86/boot/msr.h
@@ -0,0 +1,28 @@
+/* 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 tracepoint/exception handling infrastructure
+ * that's not available here, hence these boot_{rd,wr}msr helpers which serve
+ * the singular purpose of wrapping the RDMSR/WRMSR instructions to reduce the
+ * need for inline assembly calls throughout the boot kernel code.
+ */
+static inline void boot_rdmsr(unsigned int msr, struct msr *m)
+{
+	asm volatile("rdmsr" : "=a" (m->l), "=d" (m->h) : "c" (msr));
+}
+
+static inline void boot_wrmsr(unsigned int msr, const struct msr *m)
+{
+	asm volatile("wrmsr" : : "c" (msr), "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 d42e6c6b47b1..65ec1965cd28 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 000000000000..1e6ec10b3a15
--- /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 */
-- 
2.25.1