Sending INIT/SIPI to APs from ap_init() resets them into 16-bit mode
to loop into sipi_entry().
To drive the APs into 32-bit mode, the SIPI vector needs:
1. A GDT descriptor reachable from 16-bit code (gdt32_descr).
2. A 32-bit entrypoint reachable from 16-bit code (ap_start32).
3. The locations of GDT and the 32-bit entrypoint.
Setting these up at compile time (like on non-EFI builds) is not
possible since EFI builds with -shared -fPIC and efistart64.S cannot
reference any absolute addresses.
Relative addressing is unavailable on 16-bit mode.
Moreover, EFI may not load the 32-bit entrypoint to be reachable from
16-bit mode.
To overcome these problems,
1. Fill the GDT descriptor at runtime after relocating
[sipi_entry-sipi_end] to lowmem. Since sipi_entry does not know the
address of this descriptor, use the last two bytes of SIPI page to
communicate it.
2. Place a call gate in the GDT to point to ap_start32.
3. Popluate sipi_entry() to lcall to ap_start32.
With this, the APs can transition to 32-bit mode and loop at a known
location.
Signed-off-by: Varad Gautam <varad.gautam@xxxxxxxx>
---
lib/x86/smp.c | 73 +++++++++++++++++++++++++++++++++++++++++++-
lib/x86/smp.h | 3 ++
x86/efi/efistart64.S | 30 +++++++++++++++++-
3 files changed, 104 insertions(+), 2 deletions(-)
diff --git a/lib/x86/smp.c b/lib/x86/smp.c
index 90f6210..360ae82 100644
--- a/lib/x86/smp.c
+++ b/lib/x86/smp.c
@@ -21,9 +21,13 @@ static volatile bool ipi_wait;
static int _cpu_count;
static atomic_t active_cpus;
extern u8 rm_trampoline, rm_trampoline_end;
-#ifdef __i386__
+#if defined(__i386__) || defined(CONFIG_EFI)
extern u8 ap_rm_gdt_descr;
#endif
+#ifdef CONFIG_EFI
+extern u8 ap_rm_gdt, ap_rm_gdt_end;
+extern u8 ap_start32;
+#endif
atomic_t cpu_online_count = { .counter = 1 };
static __attribute__((used)) void ipi(void)
@@ -158,6 +162,73 @@ static void setup_rm_gdt(void)
* the vector code.
*/
sgdt(rm_gdt);
+#elif defined(CONFIG_EFI)
+ /*
+ * The realmode trampoline on EFI has the following layout:
+ *
+ * |rm_trampoline:
+ * |sipi_entry:
+ * | <AP bootstrapping code called from SIPI>
+ * |ap_rm_gdt:
+ * | <GDT used for 16-bit -> 32-bit trasition>
+ * |ap_rm_gdt_descr:
+ * | <GDT descriptor for ap_rm_gdt>
+ * |sipi_end:
+ * | <End of trampoline>
+ * |rm_trampoline_end:
+ *
+ * After relocating to the lowmem address pointed to by realmode_trampoline,
+ * the realmode GDT descriptor needs to contain the relocated address of
+ * ap_rm_gdt.
+ */
+ volatile struct descriptor_table_ptr *rm_gdt_descr =
+ (struct descriptor_table_ptr *) (&ap_rm_gdt_descr - &rm_trampoline);
+ rm_gdt_descr->base = (ulong) ((u32) (&ap_rm_gdt - &rm_trampoline));
+ rm_gdt_descr->limit = (u16) (&ap_rm_gdt_end - &ap_rm_gdt - 1);
+
+ /*
+ * Since 1. compile time calculation of offsets is not allowed when
+ * building with -shared, and 2. rip-relative addressing is not supported in
+ * 16-bit mode, the relocated address of ap_rm_gdt_descr needs to be stored at
+ * a location known to / accessible from the trampoline.
+ *
+ * Use the last two bytes of the trampoline page (REALMODE_GDT_LOWMEM) to store
+ * a pointer to relocated ap_rm_gdt_descr addr. This way, the trampoline code can
+ * find the relocated descriptor using the lowmem address at pa=REALMODE_GDT_LOWMEM,
+ * and this relocated descriptor points to the relocated GDT.
+ */
+ *((u16 *)(REALMODE_GDT_LOWMEM)) = (u16) (u64) rm_gdt_descr;
+
+ /*
+ * Set up a call gate to the 32-bit entrypoint (ap_start32) within GDT, since
+ * EFI may not load the 32-bit AP entrypoint (ap_start32) low enough
+ * to be reachable from the SIPI vector.
+ *
+ * Since kvm-unit-tests builds with -shared, this location needs to be fetched
+ * at runtime, and rip-relative addressing is not supported in 16-bit mode. This
+ * prevents using a long jump to ap_start32 (`ljmpl $cs, $ap_start32`).
+ *
+ * As an alternative, a far return via `push $cs; push $label; lret` would require
+ * an intermediate trampoline since $label must still be within 0 - 0xFFFF for
+ * 16-bit far return to work.
+ *
+ * Using a call gate allows for an easier 16-bit -> 32-bit transition via `lcall`.
+ *
+ * GDT layout:
+ *
+ * Entry | Segment
+ * 0 | NULL descr
+ * 1 | Code segment descr
+ * 2 | Data segment descr
+ * 3 | Call gate descr
+ *
+ * This layout is only used for reaching 32-bit mode. APs load a 64-bit GDT
+ * later during boot, which does not need to follow this layout.
+ */
+ idt_entry_t *gate_descr = (idt_entry_t *) ((u8 *)(&ap_rm_gdt - &rm_trampoline)
+ + 3 * sizeof(gdt_entry_t));
+ set_desc_entry(gate_descr, sizeof(gdt_entry_t), (void *) &ap_start32,
+ 0x8 /* sel */, 0xc /* type */, 0 /* dpl */);
#endif
}
diff --git a/lib/x86/smp.h b/lib/x86/smp.h
index 77f8a19..b805be5 100644
--- a/lib/x86/smp.h
+++ b/lib/x86/smp.h
@@ -6,6 +6,9 @@
#include "libcflat.h"
#include "atomic.h"
+/* Address where to store the address of realmode GDT descriptor. */
+#define REALMODE_GDT_LOWMEM (PAGE_SIZE - 2)
+
/* Offsets into the per-cpu page. */
struct percpu_data {
uint32_t smp_id;
diff --git a/x86/efi/efistart64.S b/x86/efi/efistart64.S
index 3a4135e..c6b1a5b 100644
--- a/x86/efi/efistart64.S
+++ b/x86/efi/efistart64.S
@@ -33,16 +33,44 @@ ptl4:
.text
.code16
+REALMODE_GDT_LOWMEM = PAGE_SIZE - 2
.globl rm_trampoline
rm_trampoline:
.globl sipi_entry
sipi_entry:
- jmp sipi_entry
+ mov %cr0, %eax
+ or $1, %eax
+ mov %eax, %cr0
+
+ /* Retrieve relocated ap_rm_gdt_descr address at REALMODE_GDT_LOWMEM. */
+ mov (REALMODE_GDT_LOWMEM), %ebx
+ lgdtl (%ebx)
+
+ lcall $0x18, $0x0
+
+.globl ap_rm_gdt
+ap_rm_gdt:
+ .quad 0
+ .quad 0x00cf9b000000ffff // flat 32-bit code segment
+ .quad 0x00cf93000000ffff // flat 32-bit data segment
+ .quad 0 // call gate to 32-bit AP entrypoint
+.globl ap_rm_gdt_end
+ap_rm_gdt_end:
+
+.globl ap_rm_gdt_descr
+ap_rm_gdt_descr:
+ .word 0
+ .long 0
.globl sipi_end
sipi_end:
.globl rm_trampoline_end
rm_trampoline_end:
+
+.code32
+.globl ap_start32
+ap_start32:
+ jmp ap_start32
--
2.32.0
