From 2d7bc823503167276c6d3c40500c3055d4f38938 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Tue, 2 Jul 2024 21:19:49 -0400
Subject: kernel/amd64: Handle the user GS segment register

This commit introduces usage of swapgs to switch out the GS register
with the user GS register. An md_td_kick() function is also introduced
to start up user threads. The this_cpu() function uses the GS register
to read the current CPU structure using a new "self" field.

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 sys/arch/amd64/amd64/machdep.c      | 20 ++++++++++++++++++--
 sys/arch/amd64/amd64/proc_machdep.c | 30 ++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+), 2 deletions(-)

(limited to 'sys/arch/amd64')

diff --git a/sys/arch/amd64/amd64/machdep.c b/sys/arch/amd64/amd64/machdep.c
index 82fa97b..528326a 100644
--- a/sys/arch/amd64/amd64/machdep.c
+++ b/sys/arch/amd64/amd64/machdep.c
@@ -95,20 +95,36 @@ try_mitigate_spectre(void)
 struct cpu_info *
 this_cpu(void)
 {
-    return (void *)amd64_read_gs_base();
+    struct cpu_info *ci;
+
+    /*
+     * This might look crazy but we are just leveraging the "m"
+     * constraint to calculate the offset of the self field within
+     * cpu_info. The self field points to the cpu_info structure
+     * itself allowing us to access cpu_info through %gs.
+     */
+    __ASMV("mov %%gs:%1, %0"
+        : "=r" (ci)
+        : "m" (*&((struct cpu_info *)0)->self)
+        : "memory");
+
+    return ci;
 }
 
 void
 cpu_startup(struct cpu_info *ci)
 {
+    ci->self = ci;
     gdt_load(&bsp_gdtr);
     idt_load();
 
     setup_vectors();
     amd64_write_gs_base((uintptr_t)ci);
-    init_tss(ci);
+    __ASMV("swapgs");           /* Get kernel GS */
 
+    init_tss(ci);
     try_mitigate_spectre();
+
     __ASMV("sti");              /* Unmask interrupts */
     lapic_init();
 }
diff --git a/sys/arch/amd64/amd64/proc_machdep.c b/sys/arch/amd64/amd64/proc_machdep.c
index 2b4368b..723582b 100644
--- a/sys/arch/amd64/amd64/proc_machdep.c
+++ b/sys/arch/amd64/amd64/proc_machdep.c
@@ -34,6 +34,7 @@
 #include <sys/exec.h>
 #include <machine/frame.h>
 #include <machine/gdt.h>
+#include <machine/cpu.h>
 #include <vm/physmem.h>
 #include <vm/vm.h>
 #include <vm/map.h>
@@ -112,6 +113,35 @@ setregs(struct proc *td, struct exec_prog *prog, uintptr_t stack)
     tfp->rflags = 0x202;
 }
 
+/*
+ * Startup a user thread.
+ *
+ * @td: Thead to start up.
+ */
+void
+md_td_kick(struct proc *td)
+{
+    struct trapframe *tfp;
+
+    tfp = &td->tf;
+    __ASMV(
+        "push %0\n"
+        "push %1\n"
+        "pushf\n"
+        "push %2\n"
+        "push %3\n"
+        "swapgs\n"
+        "iretq"
+        :
+        : "i" (USER_DS | 3),
+          "r" (tfp->rsp),
+          "i" (USER_CS | 3),
+          "r" (tfp->rip)
+    );
+
+    __builtin_unreachable();
+}
+
 /*
  * MD thread init code
  *
-- 
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