Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

[Yang, Weijiang]

On 5/17/2024 12:58 AM, Sean Christopherson wrote:
> On Thu, May 16, 2024, Dave Hansen wrote:
> > On 5/16/24 07:39, Sean Christopherson wrote:
> > > > We synced the issue internally, and got conclusion that KVM should honor host
> > > > IBT config.  In this case IBT bit in boot_cpu_data should be honored.  With
> > > > this policy, it can avoid CPUID confusion to guest side due to host ibt=off
> > > > config.
> > > What was the reasoning?  CPUID confusion is a weak justification, e.g. it's not
> > > like the guest has visibility into the host kernel, and raw CPUID will still show
> > > IBT support in the host.
> > I'm basically arguing for the path of least resistance (at least to start).
> >
> > We should just do what takes the least amount of code for now that
> > results in mostly sane behavior, then debate about making it perfect later.
> >
> > In other words, let's say the place we'd *IDEALLY* end up is that guests
> > can have any random FPU state which is disconnected from the host.  But
> > the reality, for now, is that the host needs to have XFEATURE_CET_USER
> > set in order to pass it into the guest and that means keeping
> > X86_FEATURE_SHSTK set.
> >
> > If you want guest XFEATURE_CET_USER, you must have host
> > X86_FEATURE_SHSTK ... for now.
> Ah, because fpu__init_system_xstate() will clear XFEATURE_CET_USER via the
> X86_FEATURE_SHSTK connection in xsave_cpuid_features.
>
> Please put something to that effect in the changelog.  "this literally won't work
> (without more changes)" is very different than us making a largely arbitrary
> decision.

So I need to remove the trick here for guest IBT, right?

Side topic:
When X86_FEATURE_SHSTK and X86_FEATURE_IBT (no ibt=off set) are available on host side, then host XFEATURE_CET_USER is enabled. In this case, we still *need* below patch: https://lore.kernel.org/all/20240219074733.122080-3-weijiang.yang@xxxxxxxxx/ to correctly enable XFEATURE_CET_USER in *guest kernel*, because VMM userspace can enable IBT alone for guest by -cpu host -shstk, am I right?