[kvm-unit-tests PATCH v3 17/20] x86/smp: Align the stack to a 16-byte boundary when invoking SMP function calls

[Sean Christopherson <seanjc@google.com>]

Align RSP to a 16-byte boundary in the IPI handler for SMP function calls
before calling into C code, as required by the x86-64 ABI.  Failure to
ensure the stack is properly aligned leads to obscure failures if a struct
(or any other object) tagged with __attribute__((aligned(16))) (or any
alignment greater than 16) is place on the stack.  E.g. VM-Enter will fail
on VMX if a vmx_msr_entry structure is placed on the stack.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 lib/x86/smp.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/lib/x86/smp.c b/lib/x86/smp.c
index 366e184c..0cd44cdc 100644
--- a/lib/x86/smp.c
+++ b/lib/x86/smp.c
@@ -58,12 +58,23 @@ static __attribute__((used)) void ipi(void)
 }
 
 asm (
-	 "ipi_entry: \n"
-	 "   call ipi \n"
-#ifndef __x86_64__
-	 "   iret"
+	"ipi_entry: \n"
+#ifdef __x86_64__
+	/*
+	 * Align the stack on a 16-byte boundary (as per x86_64 ABI) before
+	 * calling into C code.  Make sure not to clobber any regs!
+	 */
+	"	push %rbp\n"
+	"	mov %rsp, %rbp\n"
+	"	and $-0x10, %rsp\n"
+#endif
+	"	call ipi\n"
+#ifdef __x86_64__
+	"	mov %rbp, %rsp\n"
+	"	pop %rbp\n"
+	"	iretq"
 #else
-	 "   iretq"
+	"	iret"
 #endif
 	 );
 
-- 
2.54.0.563.g4f69b47b94-goog