Date:   Sat, 12 Oct 2019 16:58:58 -0700
In-Reply-To: <20191012235859.238387-1-morbo@google.com>
Message-Id: <20191012235859.238387-2-morbo@google.com>
Mime-Version: 1.0
References: <20191012235859.238387-1-morbo@google.com>
Subject: [kvm-unit-tests PATCH 1/2] x86: realmode: explicitly copy structure
 to avoid memcpy
From:   Bill Wendling <morbo@google.com>
To:     kvm@vger.kernel.org, pbonzini@redhat.com, alexandru.elisei@arm.com
Cc:     jmattson@google.com, Bill Wendling <morbo@google.com>
Content-Type: text/plain; charset="UTF-8"
Sender: kvm-owner@vger.kernel.org
Precedence: bulk

Clang prefers to use a "mempcy" (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. GCC performs a field-by-field copy of the
structure, so force clang to do the same thing.

Signed-off-by: Bill Wendling <morbo@google.com>
---
 x86/realmode.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/x86/realmode.c b/x86/realmode.c
index 303d093..cf45fd6 100644
--- a/x86/realmode.c
+++ b/x86/realmode.c
@@ -117,6 +117,19 @@ struct regs {
 	u32 eip, eflags;
 };
 
+#define COPY_REG(name, dst, src) (dst).name = (src).name
+#define COPY_REGS(dst, src)				\
+	COPY_REG(eax, dst, src);			\
+	COPY_REG(ebx, dst, src);			\
+	COPY_REG(ecx, dst, src);			\
+	COPY_REG(edx, dst, src);			\
+	COPY_REG(esi, dst, src);			\
+	COPY_REG(edi, dst, src);			\
+	COPY_REG(esp, dst, src);			\
+	COPY_REG(ebp, dst, src);			\
+	COPY_REG(eip, dst, src);			\
+	COPY_REG(eflags, dst, src)
+
 struct table_descr {
 	u16 limit;
 	void *base;
@@ -148,11 +161,11 @@ 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 +209,7 @@ 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