Re: [PATCH v7 4/7] KVM: VMX: Load Guest CET via VMCS when CET is enabled in Guest

[Yang Weijiang]

On Thu, Oct 10, 2019 at 04:44:17PM -0700, Jim Mattson wrote:
> On Wed, Oct 9, 2019 at 6:28 PM Yang Weijiang <weijiang.yang@xxxxxxxxx> wrote:
> >
> > On Wed, Oct 09, 2019 at 04:08:50PM -0700, Jim Mattson wrote:
> > > On Tue, Oct 8, 2019 at 11:41 PM Yang Weijiang <weijiang.yang@xxxxxxxxx> wrote:
> > > >
> > > > On Wed, Oct 02, 2019 at 11:54:26AM -0700, Jim Mattson wrote:
> > > > > On Thu, Sep 26, 2019 at 7:17 PM Yang Weijiang <weijiang.yang@xxxxxxxxx> wrote:
> > > > > > +       if (cet_on)
> > > > > > +               vmcs_set_bits(VM_ENTRY_CONTROLS,
> > > > > > +                             VM_ENTRY_LOAD_GUEST_CET_STATE);
> > > > >
> > > > > Have we ensured that this VM-entry control is supported on the platform?
> > > > >
> > > > If all the checks pass, is it enought to ensure the control bit supported?
> > >
> > > I don't think so. The only way to check to see if a VM-entry control
> > > is supported is to check the relevant VMX capability MSR.
> > >
> > It's a bit odd, there's no relevant CET bit in VMX cap. MSR, so I have
> > to check like this.
> 
> Bit 52 of the IA32_VMX_ENTRY_CTLS MSR (index 484H) [and bit 52 of the
> IA32_VMX_TRUE_ENTRY_CTLS MSR (index 490H), on hardware that supports
> the "true" VMX capability MSRs] will be 1 if it is legal to set bit 20
> of the VM-entry controls field to 1.
>
Oh, you meant this, I'll add the check, thanks.

> > > BTW, what about the corresponding VM-exit control?
> > The kernel supervisor mode CET is not implemented yet, so I don't load host CET
> > states on VM-exit, in future, I'll add it.
> 
> If you don't clear the supervisor mode CET state on VM-exit and the
> guest has set IA32_S_CET.SH_STK_EN, doesn't that mean that
> supervisor-mode shadow stacks will then be enabled on the host after
> VM-exit?

Yeah, I should clear the MSR on VM-exit before supervisor-mode is
implemented. Thank you!

BTW, I'll move this bit-set before VM-entry, vmx_set_cr4() is not a
suitable place to set the bit.