Re: [PATCH 02/13] x86/microcode: Use own MSR accessors

[Borislav Petkov]

On Tue, Jan 17, 2017 at 08:12:24PM +0100, Thomas Gleixner wrote:
> We really want to have a single place for all of this MSR stuff, because
> following your argumentation will be a perfect precedence for more private
> implementations.

Yes, you're so right. Ok, how about this untested thing. The warning
should be saucy enough :-)

---
From: Borislav Petkov <bp@xxxxxxx>
Date: Tue, 17 Jan 2017 23:29:10 +0100
Subject: [PATCH] x86/MSR: Carve out bare minimum accessors

Add __rdmsr() and __wrmsr() which *only* read and write an MSR with
exception handling. Those are going to be used in early code, like the
microcode loader, which cannot stomach tracing code piggybacking on the
MSR operation.

While at it, get rid of __native_write_msr_notrace().

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
 arch/x86/include/asm/apic.h |  2 +-
 arch/x86/include/asm/msr.h  | 51 +++++++++++++++++++++++++++------------------
 2 files changed, 32 insertions(+), 21 deletions(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 0c5fbc68e82d..eff8e36aaf72 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -195,7 +195,7 @@ static inline void native_apic_msr_write(u32 reg, u32 v)
 
 static inline void native_apic_msr_eoi_write(u32 reg, u32 v)
 {
-	wrmsr_notrace(APIC_BASE_MSR + (APIC_EOI >> 4), APIC_EOI_ACK, 0);
+	__wrmsr(APIC_BASE_MSR + (APIC_EOI >> 4), APIC_EOI_ACK, 0);
 }
 
 static inline u32 native_apic_msr_read(u32 reg)
diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index db0b90c3b03e..898dba2e2e2c 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -80,7 +80,14 @@ static inline void do_trace_read_msr(unsigned int msr, u64 val, int failed) {}
 static inline void do_trace_rdpmc(unsigned int msr, u64 val, int failed) {}
 #endif
 
-static inline unsigned long long native_read_msr(unsigned int msr)
+/*
+ * __rdmsr() and __wrmsr() are the two primitives which are the bare minimum MSR
+ * accessors and should not have any tracing or other functionality piggybacking
+ * on them - those are *purely* for accessing MSRs and nothing more. So don't even
+ * think of extending them - you will be slapped with a stinking trout or a frozen
+ * shark will reach you, wherever you are! You've been warned.
+ */
+static inline unsigned long long notrace __rdmsr(unsigned int msr)
 {
 	DECLARE_ARGS(val, low, high);
 
@@ -88,11 +95,30 @@ static inline unsigned long long native_read_msr(unsigned int msr)
 		     "2:\n"
 		     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_rdmsr_unsafe)
 		     : EAX_EDX_RET(val, low, high) : "c" (msr));
-	if (msr_tracepoint_active(__tracepoint_read_msr))
-		do_trace_read_msr(msr, EAX_EDX_VAL(val, low, high), 0);
+
 	return EAX_EDX_VAL(val, low, high);
 }
 
+static inline void notrace __wrmsr(unsigned int msr, u32 low, u32 high)
+{
+	asm volatile("1: wrmsr\n"
+		     "2:\n"
+		     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wrmsr_unsafe)
+		     : : "c" (msr), "a"(low), "d" (high) : "memory");
+}
+
+static inline unsigned long long native_read_msr(unsigned int msr)
+{
+	unsigned long long val;
+
+	val = __rdmsr(msr);
+
+	if (msr_tracepoint_active(__tracepoint_read_msr))
+		do_trace_read_msr(msr, val, 0);
+
+	return val;
+}
+
 static inline unsigned long long native_read_msr_safe(unsigned int msr,
 						      int *err)
 {
@@ -116,29 +142,14 @@ static inline unsigned long long native_read_msr_safe(unsigned int msr,
 
 /* Can be uninlined because referenced by paravirt */
 static inline void notrace
-__native_write_msr_notrace(unsigned int msr, u32 low, u32 high)
-{
-	asm volatile("1: wrmsr\n"
-		     "2:\n"
-		     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wrmsr_unsafe)
-		     : : "c" (msr), "a"(low), "d" (high) : "memory");
-}
-
-/* Can be uninlined because referenced by paravirt */
-static inline void notrace
 native_write_msr(unsigned int msr, u32 low, u32 high)
 {
-	__native_write_msr_notrace(msr, low, high);
+	__wrmsr(msr, low, high);
+
 	if (msr_tracepoint_active(__tracepoint_write_msr))
 		do_trace_write_msr(msr, ((u64)high << 32 | low), 0);
 }
 
-static inline void
-wrmsr_notrace(unsigned int msr, u32 low, u32 high)
-{
-	__native_write_msr_notrace(msr, low, high);
-}
-
 /* Can be uninlined because referenced by paravirt */
 static inline int notrace
 native_write_msr_safe(unsigned int msr, u32 low, u32 high)
-- 
2.11.0

-- 
Regards/Gruss,
    Boris.

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