Re: [PATCH v4 15/20] KVM:VMX: Save host MSR_IA32_S_CET to VMCS field

[Yang, Weijiang]

On 7/26/2023 10:05 PM, Sean Christopherson wrote:
> On Wed, Jul 26, 2023, Chao Gao wrote:
> > On Thu, Jul 20, 2023 at 11:03:47PM -0400, Yang Weijiang wrote:
> > > Save host MSR_IA32_S_CET to VMCS field as host constant state.
> > > Kernel IBT is supported now and the setting in MSR_IA32_S_CET
> > > is static after post-boot except in BIOS call case, but vCPU
> > > won't execute such BIOS call path currently, so it's safe to
> > > make the MSR as host constant.
> > >
> > > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > > ---
> > > arch/x86/kvm/vmx/capabilities.h | 4 ++++
> > > arch/x86/kvm/vmx/vmx.c          | 8 ++++++++
> > > 2 files changed, 12 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> > > index d0abee35d7ba..b1883f6c08eb 100644
> > > --- a/arch/x86/kvm/vmx/capabilities.h
> > > +++ b/arch/x86/kvm/vmx/capabilities.h
> > > @@ -106,6 +106,10 @@ static inline bool cpu_has_load_perf_global_ctrl(void)
> > > 	return vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL;
> > > }
> > >
> > > +static inline bool cpu_has_load_cet_ctrl(void)
> > > +{
> > > +	return (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_CET_STATE);
> > VM_ENTRY_LOAD_CET_STATE is to load guest state. Strictly speaking, you
> > should check VM_EXIT_LOAD_HOST_CET_STATE though I believe CPUs will
> > support both or none.
> No need, pairs are now handled by setup_vmcs_config().  See commit f5a81d0eb01e
> ("KVM: VMX: Sanitize VM-Entry/VM-Exit control pairs at kvm_intel load time"), and
> then patch 17 does:
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3eb4fe9c9ab6..3f2f966e327d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2641,6 +2641,7 @@ static int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>                  { VM_ENTRY_LOAD_IA32_EFER,              VM_EXIT_LOAD_IA32_EFER },
>                  { VM_ENTRY_LOAD_BNDCFGS,                VM_EXIT_CLEAR_BNDCFGS },
>                  { VM_ENTRY_LOAD_IA32_RTIT_CTL,          VM_EXIT_CLEAR_IA32_RTIT_CTL },
> +               { VM_ENTRY_LOAD_CET_STATE,              VM_EXIT_LOAD_CET_STATE },
>          };
>
> > > +}
> > > static inline bool cpu_has_vmx_mpx(void)
> > > {
> > > 	return vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_BNDCFGS;
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 85cb7e748a89..cba24acf1a7a 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -109,6 +109,8 @@ module_param(enable_apicv, bool, S_IRUGO);
> > > bool __read_mostly enable_ipiv = true;
> > > module_param(enable_ipiv, bool, 0444);
> > >
> > > +static u64 __read_mostly host_s_cet;
> > caching host's value is to save an MSR read on vCPU creation?
> Yep.  And probably more importantly, to document that the host value is static,
> i.e. that KVM doesn't need to refresh S_CET before every VM-Enter/VM-Exit sequence.

OK, will add it to change log, thanks!