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    SubjectRe: [PATCH 19/19] KVM: x86: Enable supervisor IBT support for guest
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    On 6/16/2022 7:19 PM, Peter Zijlstra wrote:
    > On Thu, Jun 16, 2022 at 04:46:43AM -0400, Yang Weijiang wrote:
    >> Mainline kernel now supports supervisor IBT for kernel code,
    >> to make s-IBT work in guest(nested guest), pass through
    >> MSR_IA32_S_CET to guest(nested guest) if host kernel and KVM
    >> enabled IBT. Note, s-IBT can work independent to host xsaves
    >> support because guest MSR_IA32_S_CET can be stored/loaded from
    >> specific VMCS field.
    >
    >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    >> index fe049d0e5ecc..c0118b33806a 100644
    >> --- a/arch/x86/kvm/x86.c
    >> +++ b/arch/x86/kvm/x86.c
    >> @@ -1463,6 +1463,7 @@ static const u32 msrs_to_save_all[] = {
    >> MSR_IA32_XFD, MSR_IA32_XFD_ERR,
    >> MSR_IA32_XSS,
    >> MSR_IA32_U_CET, MSR_IA32_PL3_SSP, MSR_KVM_GUEST_SSP,
    >> + MSR_IA32_S_CET,
    >> };
    >
    > So much like my local kvm/qemu hacks; this patch suffers the problem of
    > not exposing S_SHSTK. What happens if a guest tries to use that?
    With current solution, I think guest kernel will hit #GP while
    reading/writing PL0_SSP.
    >
    > Should we intercept and reject setting those bits or complete this patch
    > and support full S_SHSTK? (with all the warts and horrors that entails)
    >
    > I don't think throwing this out in this half-finished state makes much
    > sense (which is why I never much shared my hacks).

    You reminded me to think over these cases even I don't have a solution now,

    thank you!

    >
    >
    >> @@ -11830,7 +11835,13 @@ int kvm_arch_hardware_setup(void *opaque)
    >> /* Update CET features now as kvm_caps.supported_xss is finalized. */
    >> if (!kvm_cet_user_supported()) {
    >> kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
    >> - kvm_cpu_cap_clear(X86_FEATURE_IBT);
    >> + /* If CET user bit is disabled due to cmdline option such as
    >> + * noxsaves, but kernel IBT is on, this means we can expose
    >> + * kernel IBT alone to guest since CET user mode msrs are not
    >> + * passed through to guest.
    >> + */
    > Invalid multi-line comment style.
    Oops, last minute change messed it up :-(
    >
    >> + if (!cet_kernel_ibt_supported())
    >> + kvm_cpu_cap_clear(X86_FEATURE_IBT);

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