Bug 241269

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Description of problem:
local_save_flags(flags) is used to obtain the processor eflags register on x86_64.  On a bare metal kernel, each of the bits in the flags register has some significance.  It's not uncommon for code to test this flags value for the X86_EFLAGS_IF bit, which indicates whether or not interrupts are currently enabled on the local cpu.  If that bit is set, then interrupts are enabled.

Typical code:

unsigned long flags;
local_save_flags(flags);

if (flags & X86_EFLAGS_IF)
   do_A();  // because interrupts are enabled
else
   do_B();  // because they aren't.

Under a xen kernel (paravirt), local_save_flags(flags) loads the flags variable with the event channel mask instead (see __raw_local_save_flags).  And this value is 1 when interrupts are masked, 0 when they are enabled.  The result of this is that the code above will make the opposite decision under a paravirt kernel than from the bare metal kernel.

Doesn't seem like it would be too hard to cobble up a simulated flags register under the paravirt kernel.  The end of __raw_local_save_flags could go like this:

if (_vcpu->evtchn_upcall_mask == 0)
   return X86_EFLAGS_IF;
else
   return 0;

That would at least allow code like the above example to run in the same way.  A couple of other xen-kernel functions would have to be changed in a similar fashion to make that work.

FWIW, most of the paravirt kernel has been purged of code that would get tripped up by this.  But what about 3rd party kernel modules?


Version-Release number of selected component (if applicable):
kernel-2.6.18-19

How reproducible:
Always


Steps to Reproduce:
1. Run the code mentioned above in both the bare-metal and paravirt kernels
2.
3.

Actual Results:
The code in a paravirt kernel believes interrupts are always disabled


Expected Results:
Be nice if the sample code behaved the same in bare-metal and paravirt kernels.

Additional info: