target/i386/translate.c: always write 32-bits for SGDT and SIDT

The various Intel CPU manuals claim that SGDT and SIDT can write either 24-bits
or 32-bits depending upon the operand size, but this is incorrect. Not only do
the Intel CPU manuals give contradictory information between processor
revisions, but this information doesn't even match real-life behaviour.

In fact, tests on real hardware show that the CPU always writes 32-bits for SGDT
and SIDT, and this behaviour is required for at least OS/2 Warp and WFW 3.11 with
Win32s to function correctly. Remove the masking applied due to the operand size
for SGDT and SIDT so that the TCG behaviour matches the behaviour on real
hardware.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2198

--
MCA: Whilst I don't have a copy of OS/2 Warp handy, I've confirmed that this
patch fixes the issue in WFW 3.11 with Win32s. For more technical information I
highly recommend the excellent write-up at
https://www.os2museum.com/wp/sgdtsidt-fiction-and-reality/.
Message-ID: <20240419195147.434894-1-mark.cave-ayland@ilande.co.uk>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 8 insertions, 6 deletions

diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
index 76a42c6..c05d9e5 100644
--- a/target/i386/tcg/translate.c
+++ b/target/i386/tcg/translate.c
@@ -5846,9 +5846,10 @@ static bool disas_insn(DisasContext *s, CPUState *cpu)
             gen_op_st_v(s, MO_16, s->T0, s->A0);
             gen_add_A0_im(s, 2);
             tcg_gen_ld_tl(s->T0, tcg_env, offsetof(CPUX86State, gdt.base));
-            if (dflag == MO_16) {
-                tcg_gen_andi_tl(s->T0, s->T0, 0xffffff);
-            }
+            /*
+             * NB: Despite a confusing description in Intel CPU documentation,
+             *     all 32-bits are written regardless of operand size.
+             */
             gen_op_st_v(s, CODE64(s) + MO_32, s->T0, s->A0);
             break;
 
@@ -5901,9 +5902,10 @@ static bool disas_insn(DisasContext *s, CPUState *cpu)
             gen_op_st_v(s, MO_16, s->T0, s->A0);
             gen_add_A0_im(s, 2);
             tcg_gen_ld_tl(s->T0, tcg_env, offsetof(CPUX86State, idt.base));
-            if (dflag == MO_16) {
-                tcg_gen_andi_tl(s->T0, s->T0, 0xffffff);
-            }
+            /*
+             * NB: Despite a confusing description in Intel CPU documentation,
+             *     all 32-bits are written regardless of operand size.
+             */
             gen_op_st_v(s, CODE64(s) + MO_32, s->T0, s->A0);
             break;