smm: Backup and restore A20 on an SMI based mode switch

QEMU does not store the A20 setting in the SMM cpu environment area
(and it does not look like real CPUs do either).  So, manually backup
and restore A20 on a mode switch.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/fw/smm.c b/src/fw/smm.c
index 95f6ba7d..178959c4 100644
--- a/src/fw/smm.c
+++ b/src/fw/smm.c
@@ -52,7 +52,8 @@ struct smm_state {
 struct smm_layout {
     struct smm_state backup1;
     struct smm_state backup2;
-    u8 stack[0x7c00];
+    u32 backup_a20;
+    u8 stack[0x8000 - sizeof(struct smm_state)*2 - sizeof(u32)];
     u64 codeentry;
     u8 pad_8008[0x7df8];
     struct smm_state cpu;
@@ -102,10 +103,13 @@ handle_smi(u16 cs)
                 memcpy(&smm->cpu, &smm->backup1, sizeof(smm->cpu));
                 memcpy(&smm->cpu.i32.eax, regs, sizeof(regs));
                 smm->cpu.i32.eip = regs[3];
+                // Enable a20 and backup its previous state
+                smm->backup_a20 = set_a20(1);
             } else if (smm->cpu.i32.ecx == CALL32SMM_RETURNID) {
                 dprintf(9, "smm cpu ret %x esp=%x\n", regs[3], regs[4]);
                 memcpy(&smm->cpu, &smm->backup2, sizeof(smm->cpu));
                 memcpy(&smm->cpu.i32.eax, regs, sizeof(regs));
+                set_a20(smm->backup_a20);
                 smm->cpu.i32.eip = regs[3];
             }
         } else if (rev == SMM_REV_I64) {
@@ -116,9 +120,12 @@ handle_smi(u16 cs)
                 memcpy(&smm->cpu, &smm->backup1, sizeof(smm->cpu));
                 memcpy(&smm->cpu.i64.rdi, regs, sizeof(regs));
                 smm->cpu.i64.rip = (u32)regs[4];
+                // Enable a20 and backup its previous state
+                smm->backup_a20 = set_a20(1);
             } else if ((u32)smm->cpu.i64.rcx == CALL32SMM_RETURNID) {
                 memcpy(&smm->cpu, &smm->backup2, sizeof(smm->cpu));
                 memcpy(&smm->cpu.i64.rdi, regs, sizeof(regs));
+                set_a20(smm->backup_a20);
                 smm->cpu.i64.rip = (u32)regs[4];
             }
         }