From e818cab3f1fc4a3d4d0b8e2c9424fb2e7b1dff70 Mon Sep 17 00:00:00 2001
From: dzwdz
Date: Fri, 2 Aug 2024 01:20:40 +0200
Subject: *: use a generic UserRegs type everywhere I'm storing registers

mostly inspired by Plan 9's Ureg, probably obvious from the name
---
 src/kernel/arch/amd64/regs.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 src/kernel/arch/amd64/regs.c

(limited to 'src/kernel/arch/amd64/regs.c')

diff --git a/src/kernel/arch/amd64/regs.c b/src/kernel/arch/amd64/regs.c
new file mode 100644
index 0000000..bb4e5aa
--- /dev/null
+++ b/src/kernel/arch/amd64/regs.c
@@ -0,0 +1,26 @@
+#include <kernel/arch/generic.h>
+#include <kernel/panic.h>
+
+void
+regs_safecopy(UserRegs *dst, const UserRegs *src)
+{
+	/* preserved flags: (see intel vol 1 3.4.3.)
+	 * 1<<0 = CF, carry flag
+	 * 1<<2 = PF, parity flag
+	 * 1<<4 = AC, auxilary carry
+	 * 1<<6 = ZF, zero flag
+	 * 1<<7 = SF, sign flag
+	 * 1<<11 = OF, overflow flag
+	 * 1<<10 = DF, direction flag
+	 */
+	static_assert(
+		((1<<0) | (1<<2) | (1<<4) | (1<<6) | (1<<7) | (1<<10) | (1<<11))
+		== 0xCD5
+	);
+
+	uint64_t newflags = (src->flags & 0xCD5) | (dst->flags & ~0xCD5);
+	/* I "spelled it out" directly instead of using memcpy hoping that gcc
+	 * maybe would notice it can skip copying the last 8 bytes. It didn't :( */
+	*dst = *src;
+	dst->flags = newflags;
+}
-- 
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