Re: [RFC PATCH v2 00/11] Intel IA32_SPEC_CTRL Virtualization

[Pawan Gupta]

On Fri, Apr 14, 2023 at 05:51:43PM +0800, Binbin Wu wrote:
> 
> On 4/14/2023 2:25 PM, Chao Gao wrote:
> > Changes since RFC v1:
> >   * add two kselftests (patch 10-11)
> >   * set virtual MSRs also on APs [Pawan]
> >   * enable "virtualize IA32_SPEC_CTRL" for L2 to prevent L2 from changing
> >     some bits of IA32_SPEC_CTRL (patch 4)
> >   * other misc cleanup and cosmetic changes
> > 
> > RFC v1: https://lore.kernel.org/lkml/20221210160046.2608762-1-chen.zhang@xxxxxxxxx/
> > 
> > 
> > This series introduces "virtualize IA32_SPEC_CTRL" support. Here are
> > introduction and use cases of this new feature.
> > 
> > ### Virtualize IA32_SPEC_CTRL
> > 
> > "Virtualize IA32_SPEC_CTRL" [1] is a new VMX feature on Intel CPUs. This feature
> > allows VMM to lock some bits of IA32_SPEC_CTRL MSR even when the MSR is
> > pass-thru'd to a guest.
> > 
> > 
> > ### Use cases of "virtualize IA32_SPEC_CTRL" [2]
> > 
> > Software mitigations like Retpoline and software BHB-clearing sequence depend on
> > CPU microarchitectures. And guest cannot know exactly the underlying
> > microarchitecture. When a guest is migrated between processors of different
> > microarchitectures, software mitigations which work perfectly on previous
> > microachitecture may be not effective on the new one. To fix the problem, some
> > hardware mitigations should be used in conjunction with software mitigations.
> 
> So even the hardware mitigations are enabled, the software mitigations are
> still needed, right?

Retpoline mitigation is not fully effective when RET can take prediction
from an alternate predictor. Newer hardware provides a way to disable
this behavior (using RRSBA_DIS_S bit in MSR SPEC_CTRL).

eIBRS is the preferred way to mitigate BTI, but for some reason when a
guest has deployed retpoline, VMM can make it more effective by
deploying the relevant hardware control. That is why the above text
says:

  "... hardware mitigations should be used in conjunction with software
  mitigations."