Re: [PATCH v3 6/8] KVM:VMX: Load Guest CET via VMCS when CET is enabled in Guest

[Sean Christopherson]

On Mon, Mar 04, 2019 at 08:36:55PM +0800, Yang Weijiang wrote:
> On Mon, Mar 04, 2019 at 07:12:02PM -0800, Sean Christopherson wrote:
> > On Mon, Mar 04, 2019 at 05:56:40PM +0800, Yang Weijiang wrote:
> > > Cannot agree with you more!
> > > This is some design limitation, but from my point of view, once vmm
> > > exposes CET capability to guest via CPUID, it grants the guest kernel freedom to choose
> > > which features to be enabled, we don't need to add extra constraints on
> > > the usage.
> > 
> > But if KVM allows SHSTK and IBT to be toggled independently then the VMM
> > has only exposed SHSTK or IBT, not CET as whole.
> > 
> > Even if SHSTK and IBT are bundled together the guest still has to opt-in
> > to enabling each feature.  I don't see what we gain by pretending that
> > SHSTK/IBT can be individually exposed to the guest, and on the flip side
> > doing so creates a virtualization hole.
> you almost convinced me ;-), maybe I'll make the feature as a bundle in
> next release after check with kernel team. BTW, what do you mean by
> saying "create a virtualization hole"? Is it what you stated in above
> reply?

By "virtualization hole" I mean the guest would be able to use a feature
that the virtual CPU model says isn't supported.

After rereading the XSS architecture, there's a marginally less crappy
option for handling XRSTOR as we could use the XSS_EXIT_BITMAP to
intercept XRSTOR if SHSTK != IBT and the guest is restoring CET state,
e.g. to ensure the guest isn't setting IA32_PL*_SSP if !SHSTK and isn't
setting bits that are effectively reserved in IA32_U_CET.

But practically speaking that'd be the same as intercepting XRSTORS
unconditionally when the guest is using CET, i.e. it's still going to
tank the performance of a guest that uses CET+XSAVES/XRSTORS.