kern/amd64: cpu: Add routine to get current core

Signed-off-by: Ian Moffett <ian@osmora.org>

3 files changed, 69 insertions, 0 deletions

diff --git a/src/sys/arch/amd64/cpu/cpu_conf.c b/src/sys/arch/amd64/cpu/cpu_conf.c
index eac82d5..9399973 100644
--- a/src/sys/arch/amd64/cpu/cpu_conf.c
+++ b/src/sys/arch/amd64/cpu/cpu_conf.c
@@ -29,9 +29,14 @@
 
 #include <sys/cpuvar.h>
 #include <machine/boot.h>
+#include <machine/msr.h>
 
 void
 cpu_conf(struct pcore *pcore)
 {
+    pcore->self = pcore;
     platform_boot();
+
+    /* We use %GS to store the processor */
+    wrmsr(IA32_GS_BASE, (uintptr_t)pcore);
 }
diff --git a/src/sys/arch/amd64/os/machdep.c b/src/sys/arch/amd64/os/machdep.c
new file mode 100644
index 0000000..f025677
--- /dev/null
+++ b/src/sys/arch/amd64/os/machdep.c
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2025 Ian Marco Moffett and L5 engineers
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the project nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <sys/cpuvar.h>
+#include <machine/msr.h>
+
+/*
+ * Get the current processing element
+ */
+struct pcore *
+this_core(void)
+{
+    struct pcore *cpu;
+
+    if (rdmsr(IA32_GS_BASE) == 0) {
+        return NULL;
+    }
+
+    /* Get GS offset by 'self' field */
+    __ASMV(
+        "mov %%gs:%1, %0"
+        : "=r" (cpu)
+        : "m" (*&((struct pcore *)0)->self)
+        : "memory"
+    );
+
+    return cpu;
+}
diff --git a/src/sys/include/sys/cpuvar.h b/src/sys/include/sys/cpuvar.h
index f208f82..b5ff31b 100644
--- a/src/sys/include/sys/cpuvar.h
+++ b/src/sys/include/sys/cpuvar.h
@@ -42,12 +42,14 @@
  *
  * @id: Monotonic logical ID
  * @md: Machine dependent processor information
+ * @self: Chain pointer to self
  */
 struct pcore {
     uint32_t id;
 #if defined(_KERNEL)
     struct mdcore md;
 #endif  /* _KERNEL */
+    struct pcore *self;
 };
 
 #if defined(_KERNEL)
@@ -59,5 +61,13 @@ struct pcore {
  * @pcore: Core to configure
  */
 void cpu_conf(struct pcore *pcore);
+
+/*
+ * Get the current processing element (core) as
+ * a 'pcore' descriptor.
+ *
+ * Returns NULL on failure.
+ */
+struct pcore *this_core(void);
 #endif  /* _KERNEL */
 #endif  /* !_SYS_CPUVAR_H_ */