target/i386: fix SP when taking a memory fault during POP

When OS/2 Warp configures its segment descriptors, many of them are configured with
the P flag clear to allow for a fault-on-demand implementation. In the case where
the stack value is POPped into the segment registers, the SP is incremented before
calling gen_helper_load_seg() to validate the segment descriptor:

IN:
0xffef2c0c:  66 07                    popl     %es

OP:
 ld_i32 loc9,env,$0xfffffffffffffff8
 sub_i32 loc9,loc9,$0x1
 brcond_i32 loc9,$0x0,lt,$L0
 st16_i32 loc9,env,$0xfffffffffffffff8
 st8_i32 $0x1,env,$0xfffffffffffffffc

 ---- 0000000000000c0c 0000000000000000
 ext16u_i64 loc0,rsp
 add_i64 loc0,loc0,ss_base
 ext32u_i64 loc0,loc0
 qemu_ld_a64_i64 loc0,loc0,noat+un+leul,5
 add_i64 loc3,rsp,$0x4
 deposit_i64 rsp,rsp,loc3,$0x0,$0x10
 extrl_i64_i32 loc5,loc0
 call load_seg,$0x0,$0,env,$0x0,loc5
 add_i64 rip,rip,$0x2
 ext16u_i64 rip,rip
 exit_tb $0x0
 set_label $L0
 exit_tb $0x7fff58000043

If helper_load_seg() generates a fault when validating the segment descriptor then as
the SP has already been incremented, the topmost word of the stack is overwritten by
the arguments pushed onto the stack by the CPU before taking the fault handler. As a
consequence things rapidly go wrong upon return from the fault handler due to the
corrupted stack.

Update the logic for the existing writeback condition so that a POP into the segment
registers also calls helper_load_seg() first before incrementing the SP, so that if a
fault occurs the SP remains unaltered.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2198
Message-ID: <20240606095319.229650-4-mark.cave-ayland@ilande.co.uk>
Fixes: cc1d28bdbe0 ("target/i386: move 00-5F opcodes to new decoder", 2024-05-07)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/tcg/emit.c.inc

@@ -2578,7 +2578,7 @@ static void gen_POP(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
     X86DecodedOp *op = &decode->op[0];
     MemOp ot = gen_pop_T0(s);
 
-    if (op->has_ea) {
+    if (op->has_ea || op->unit == X86_OP_SEG) {
         /* NOTE: order is important for MMU exceptions */
         gen_writeback(s, decode, 0, s->T0);
     }