[Qemu-devel] Fix for incorrect SYSRET instruction implementation -- anyone looked at this yet?

[Bill Paul <wpaul@windriver.com>]

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Nobody has commented on this yet. According to my reading of the Intel 
documentation, the SYSRET instruction is supposed to force the RPL bits of the 
%ss register to 3 when returning to user mode. The actual sequence is:

SS.Selector <-- (IA32_STAR[63:48]+8) OR 3; (* RPL forced to 3 *)

However, the code in helper_sysret() leaves them at 0 (in other words, the "OR 
3" part of the above sequence is missing). It does set the privilege level 
bits of %cs correctly though.

This has caused me trouble with some of my VxWorks development: code that runs 
okay on real hardware will crash on QEMU, unless I apply the patch below.

Can someone confirm that this is in fact a real bug? The Intel architecture 
manual seems quite clear about the SYSRET behavior. The bug seems to have been 
around as far back as QEMU 0.10.5.

I am using QEMU 2.2.0 on FreeBSD/amd64 9.1-RELEASE.

-Bill

Signed-off-by: Bill Paul <wpaul@windriver.com>

---
 target-i386/seg_helper.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/target-i386/seg_helper.c b/target-i386/seg_helper.c
index fa374d0..2bc757a 100644
--- a/target-i386/seg_helper.c
+++ b/target-i386/seg_helper.c
@@ -1043,7 +1043,7 @@ void helper_sysret(CPUX86State *env, int dflag)
                                    DESC_CS_MASK | DESC_R_MASK | DESC_A_MASK);
             env->eip = (uint32_t)env->regs[R_ECX];
         }
-        cpu_x86_load_seg_cache(env, R_SS, selector + 8,
+        cpu_x86_load_seg_cache(env, R_SS, (selector + 8) | 3,
                                0, 0xffffffff,
                                DESC_G_MASK | DESC_B_MASK | DESC_P_MASK |
                                DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
@@ -1056,7 +1056,7 @@ void helper_sysret(CPUX86State *env, int dflag)
                                DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
                                DESC_CS_MASK | DESC_R_MASK | DESC_A_MASK);
         env->eip = (uint32_t)env->regs[R_ECX];
-        cpu_x86_load_seg_cache(env, R_SS, selector + 8,
+        cpu_x86_load_seg_cache(env, R_SS, (selector + 8) | 3,
                                0, 0xffffffff,
                                DESC_G_MASK | DESC_B_MASK | DESC_P_MASK |
                                DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
-- 
1.8.0

-- 
=============================================================================
-Bill Paul            (510) 749-2329 | Senior Member of Technical Staff,
                 wpaul@windriver.com | Master of Unix-Fu - Wind River Systems
=============================================================================
   "I put a dollar in a change machine. Nothing changed." - George Carlin
=============================================================================

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