Re: [Qemu-devel] CC_DST problem

[Fabrice Bellard <fabrice@bellard.org>]

Alexander Graf wrote:
> Hi,
> 
> I'm still trying to implement SVM correctly and hit a serious problem.
> If I set CC_OP to EFLAGS / DYNAMIC after each instruction (so most
> conditional operations are based on EFLAGS) everything works as expected.
> If using CC_OP==CC_OP_EFLAGS only CC_SRC should be used and CC_DST is
> supposed to be completely ignored.
> 
> So I set CC_DST to 0 (this happens when leaving and rejoining the
> virtual machine, so this is the real problem) and if I do that, I get
> funny segmentation faults in x86_64 guest userspace programs running in
> the virtual machine (this is exactly what I see in kvm with my current
> patchset as well), while 32 bit userspace programs simply hang.
> So I guess this is the real problem.
> 
> Is there any logical reason CC_DST could be used with CC_OP==CC_OP_EFLAGS?
> 
> Attached to this email you will find a small patch that triggers this
> problem.
> 
> Thanks for any reply that could help on this,
> 
> Alexander Graf

If you play with the CC_OP logic, it is better to disable the eflags 
optimization code in the translator (optimize_flags() function).

Regarding the implementation for SVM, you can look at how the CC are 
handled in SMM (do_smm_enter and helper_rsm). I see no particular 
problem here.

I suggest to try to suppress the additions in the static translator 
state as I feel most of the SVM intercepts can be tested in helpers 
where speed is not critical.

Regards,

Fabrice.