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>

diff --git a/src/fw/smm.c b/src/fw/smm.c
index 95f6ba7d9a09a0c2958dfbc462a76853fda6fa5c..178959c41391ca512effdc3721a8817c80c75c2f 100644 (file)
--- 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];
             }
         }