core: pxe: don't switch to the PXE stack if we're on it...

Don't switch to the PXE stack if we are already on it.  This can
happen if we take an interrupt inside the stack switch code.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/core/pxelinux.asm b/core/pxelinux.asm
index ab8526a..be304be 100644 (file)
--- a/core/pxelinux.asm
+++ b/core/pxelinux.asm
@@ -375,9 +375,15 @@ pxenv:
                call timer_cleanup
 
 .store_stack:
+               pushf
+               cli
+               inc word [cs:PXEStackLock]
+               jnz .skip1
                mov [cs:PXEStack],sp
                mov [cs:PXEStack+2],ss
                lss sp,[cs:InitStack]
+.skip1:
+               popf
 
                ; Pre-clear the Status field
                mov word [es:di],cs
@@ -392,7 +398,13 @@ pxenv:
                add sp,6
                mov [cs:PXEStatus],ax
 
+               pushf
+               cli
+               dec word [cs:PXEStackLock]
+               jns .skip2
                lss sp,[cs:PXEStack]
+.skip2:
+               popf
 
                mov bp,sp
                and ax,ax
@@ -419,6 +431,16 @@ pxenv:
                 global PXEEntry
 PXEEntry       equ pxenv.jump+1
 
+;
+; The PXEStackLock keeps us from switching stacks if we take an interrupt
+; (which ends up calling pxenv) while we are already on the PXE stack.
+; It will be -1 normally, 0 inside a PXE call, and a positive value
+; inside a *nested* PXE call.
+;
+               section .data16
+               alignb 2
+PXEStackLock   dw -1
+
                section .bss16
                alignb 2
 PXEStatus      resb 2