[kvm-unit-tests v2 PATCH 1/2] x86: realmode: explicitly copy regs structure

[Bill Wendling <morbo@google.com>]

Clang prefers to use a "rep;movsl" (or equivalent) to copy the "regs"
structure. This doesn't work in 16-bit mode, as it will end up copying
over half the number of bytes. Avoid this by copying over the structure
a byte at a time.

Signed-off-by: Bill Wendling <morbo@google.com>
---
 x86/realmode.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/x86/realmode.c b/x86/realmode.c
index 303d093..41b8592 100644
--- a/x86/realmode.c
+++ b/x86/realmode.c
@@ -140,6 +140,16 @@ struct insn_desc {
 
 static struct regs inregs, outregs;
 
+static inline void copy_regs(struct regs *dst_regs, struct regs *src_regs)
+{
+	char *dst = (char*)dst_regs;
+	char *src = (char*)src_regs;
+	u32 i;
+
+	for (i = 0; i < sizeof(struct regs); i++)
+		dst[i] = src[i];
+}
+
 static void exec_in_big_real_mode(struct insn_desc *insn)
 {
 	unsigned long tmp;
@@ -148,11 +158,11 @@ static void exec_in_big_real_mode(struct insn_desc *insn)
 	extern u8 test_insn[], test_insn_end[];
 
 	for (i = 0; i < insn->len; ++i)
-	    test_insn[i] = ((u8 *)(unsigned long)insn->ptr)[i];
+		test_insn[i] = ((u8 *)(unsigned long)insn->ptr)[i];
 	for (; i < test_insn_end - test_insn; ++i)
 		test_insn[i] = 0x90; // nop
 
-	save = inregs;
+	copy_regs(&save, &inregs);
 	asm volatile(
 		"lgdtl %[gdt_descr] \n\t"
 		"mov %%cr0, %[tmp] \n\t"
@@ -196,7 +206,8 @@ static void exec_in_big_real_mode(struct insn_desc *insn)
 		: [gdt_descr]"m"(gdt_descr), [bigseg]"r"((short)16)
 		: "cc", "memory"
 		);
-	outregs = save;
+	copy_regs(&outregs, &save);
+
 }
 
 #define R_AX 1
-- 
2.23.0.700.g56cf767bdb-goog