[PATCH v2] target/i386: seg_helper: Correct segement selector nullification in the RET/IRET helper

[Bin Meng <bmeng.cn@gmail.com>]

From: Bin Meng <bin.meng@windriver.com>

Per the SDM, when returning to outer privilege level, for segment
registers (ES, FS, GS, and DS) if the check fails, the segment
selector becomes null, but QEMU clears the base/limit/flags as well
as nullifying the segment selector, which should be a spec violation.

Real hardware seems to be compliant with the spec, at least on one
Coffee Lake board I tested.

Signed-off-by: Bin Meng <bin.meng@windriver.com>

---

Changes in v2:
- clearing the DESC_P bit in the segment descriptor

 target/i386/seg_helper.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/target/i386/seg_helper.c b/target/i386/seg_helper.c
index be88938..d539573 100644
--- a/target/i386/seg_helper.c
+++ b/target/i386/seg_helper.c
@@ -2108,7 +2108,10 @@ static inline void validate_seg(CPUX86State *env, int seg_reg, int cpl)
     if (!(e2 & DESC_CS_MASK) || !(e2 & DESC_C_MASK)) {
         /* data or non conforming code segment */
         if (dpl < cpl) {
-            cpu_x86_load_seg_cache(env, seg_reg, 0, 0, 0, 0);
+            cpu_x86_load_seg_cache(env, seg_reg, 0,
+                                   env->segs[seg_reg].base,
+                                   env->segs[seg_reg].limit,
+                                   env->segs[seg_reg].flags & ~DESC_P_MASK);
         }
     }
 }
-- 
2.7.4