[PATCH]Re: [Qemu-devel] [AMD64] Possible problem with the DF flag during the SYSCALL instruction

[Jakub Jermar]

Jakub Jermar wrote:
> yesterday I tried to mask off the DF flag from the RFLAGS register 
> during the SYSCALL instruction by specifying this bit in the SFMASK MSR 
> register while running HelenOS.
>
> With QEMU, this didn't work (but it worked in Simics, for instance), so 
> I suspect that there is something wrong with handling RFLAGS.DF during 
> SYSCALL. Anyone wants to have a look?

I have looked into the problem a little more and found out that the
culprit here is probably a missing update of the DF variable in the
helper_syscall() function, in target-i386/helper.c.  See the attached
patch.

The patch fixes the problem for me, but is probably not complete (i.e.
there might be other places that need a similar treatment), but I can't
really tell since I am not a qemu expert.

Jakub

--- helper.c    2008-01-06 20:38:45.000000000 +0100
+++ helper.c.new        2008-03-20 22:39:17.000000000 +0100
@@ -1008,6 +1008,7 @@
                                DESC_S_MASK |
                                DESC_W_MASK | DESC_A_MASK);
         env->eflags &= ~env->fmask;
+       DF = 1 - (2 * ((env->eflags >> 10) & 1));
         if (code64)
             env->eip = env->lstar;
         else