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[PATCH]Re: [Qemu-devel] [AMD64] Possible problem with the DF flag during
[PATCH]Re: [Qemu-devel] [AMD64] Possible problem with the DF flag during the SYSCALL instruction
Thu, 20 Mar 2008 22:46:10 +0100
Thunderbird 126.96.36.199 (X11/20080227)
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
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.
--- 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_W_MASK | DESC_A_MASK);
env->eflags &= ~env->fmask;
+ DF = 1 - (2 * ((env->eflags >> 10) & 1));
env->eip = env->lstar;