[PATCH] x86/asm: Force native_apic_mem_read to use mov

[Adam Dunlap <acdunlap@google.com>]

Previously, when compiled with clang, native_apic_mem_read gets inlined
into __xapic_wait_icr_idle and optimized to a testl instruction. When
run in a VM with SEV-ES enabled, it attempts to emulate this
instruction, but the emulator does not support it. Instead, use inline
assembly to force native_apic_mem_read to use the mov instruction which
is supported by the emulator.

Signed-off-by: Adam Dunlap <acdunlap@google.com>
Reviewed-by: Marc Orr <marcorr@google.com>
Reviewed-by: Jacob Xu <jacobhxu@google.com>
---
 arch/x86/include/asm/apic.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 3415321c8240..281db79e76a9 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -109,7 +109,18 @@ static inline void native_apic_mem_write(u32 reg, u32 v)
 
 static inline u32 native_apic_mem_read(u32 reg)
 {
-	return *((volatile u32 *)(APIC_BASE + reg));
+	volatile u32 *addr = (volatile u32 *)(APIC_BASE + reg);
+	u32 out;
+
+	/*
+	 * Functionally, what we want to do is simply return *addr. However,
+	 * this accesses an MMIO which may need to be emulated in some cases.
+	 * The emulator doesn't necessarily support all instructions, so we
+	 * force the read from addr to use a mov instruction.
+	 */
+	asm_inline("movl %1, %0" : "=r"(out) : "m"(*addr));
+
+	return out;
 }
 
 extern void native_apic_wait_icr_idle(void);
-- 
2.37.1.559.g78731f0fdb-goog