----- ar...@linux.intel.com wrote:
> On 1/9/2018 3:41 AM, Paolo Bonzini wrote:
> > The above ("IBRS simply disables the indirect branch predictor") was
> my
> > take-away message from private discussion with Intel. My guess is
> that
> > the vendors are just handwaving a spec that doesn't match what they
> have
> > implemented, because honestly a microcode update is unlikely to do
> much
> > more than an old-fashioned chicken bit. Maybe on Skylake it does
> > though, since the performance characteristics of IBRS are so
> different
> > from previous processors. Let's ask Arjan who might have more
> > information about it, and hope he actually can disclose it...
>
> IBRS will ensure that, when set after the ring transition, no earlier
> branch prediction data is used for indirect branches while IBRS is
> set
Consider the following scenario:
1. L1 runs with IBRS=1 in Ring0.
2. L1 restores L2 SPEC_CTRL and enters into L2.
3. L1 VMRUN exits into L0 which backups L1 SPEC_CTRL and enters L2 (using same
VMCB).
4. L2 populates BTB/BHB with values and cause a hypercall which #VMExit into L0.
5. L0 backups L2 SPEC_CTRL and writes IBRS=1.
6. L0 restores L1 SPEC_CTRL and enters L1.
7. L1 backups L2 SPEC_CTRL and writes IBRS=1.
Just to make sure I understand:
You state that L2 BTB/BHB won't be used by L1 because L1 have set IBRS=1 in
step (7).
And that is even though L1 & L2 could both be running in SVM guest-mode & Ring0
from physical CPU perspective. Therefore, having the same prediction-mode.
So basically you are saying that IBRS don't make sure to avoid using BTB/BHB
from lower prediction-modes but instead just make sure to avoid usage of all
BTB/BHB while IBRS is set.
Did I understand everything correctly?
Thanks,
-Liran
>
> (this is a english summary of two pages of technical spec so it lacks
> the language lawyer precision)
>
> because of this promise, the implementation tends to be impactful
> and it is very strongly recommended that retpoline is used instead of
> IBRS.
> (with all the caveats already on lkml)
>
> the IBPB is different, this is a covenient thing for switching between
> VM guests etc
