On Wed, Aug 11, 2021, Chenyi Qiang wrote:
> @@ -7207,6 +7257,19 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>
> /* Refresh #PF interception to account for MAXPHYADDR changes. */
> vmx_update_exception_bitmap(vcpu);
> +
> + if (kvm_cpu_cap_has(X86_FEATURE_PKS) &&
> + guest_cpuid_has(vcpu, X86_FEATURE_PKS)) {
Ah, this confused me for a second. It's not wrong to clear the entry/exit controls
in the "else" path, but it's surprisingly hard to follow because it reads as if the
entry/exit controls are paired with the MSR behavior.
Oh, and more importantly, it's "hiding" a bug: the MSR bitmap needs to be _set_
if userspace disables X86_FEATURE_PKS in guest CPUID, e.g. if for some reason
userspace exposed PKS and then yanked it away.
Oof, two bugs actually. This will fail to re-enable the entry/exit bits if
userspace hides PKS and then re-enables PKS.
Heh, make that three bugs. If userspace never sets CPUID, KVM will run with
the entry/exit bits set. That's arguably not a bug since functionally it's fine,
but it's a bug in the sense that KVM loads an MSR when it doesn't inted to do so.
So this should be:
if (kvm_vcpu_cap_has(X86_FEATURE_PKS) {
if (guest_cpuid_has(vcpu, X86_FEATURE_PKS)) {
vmx_disable_intercept_for_msr(vcpu, MSR_IA32_PKRS, MSR_TYPE_RW);
vm_entry_controls_setbit(vmx, VM_ENTRY_LOAD_IA32_PKRS);
vm_exit_controls_setbit(vmx, VM_EXIT_LOAD_IA32_PKRS)
} else {
vmx_enable_intercept_for_msr(vcpu, MSR_IA32_PKRS, MSR_TYPE_RW);
vm_entry_controls_clearbit(vmx, VM_ENTRY_LOAD_IA32_PKRS);
vm_exit_controls_clearbit(vmx, VM_EXIT_LOAD_IA32_PKRS)
}
}
and then the bits need to be masked in vmx_vmexit_ctrl() and vmx_vmentry_ctrl(),
a la EFER and PERF_GLOBAL_CTRL.
> + vmx_disable_intercept_for_msr(vcpu, MSR_IA32_PKRS, MSR_TYPE_RW);
> + } else {
> + /*
> + * Remove VM control in case guest VM doesn't support PKS to mitigate
> + * overhead during VM-{exit,entry}. They are present by default
> + * if supported.
> + */
> + vm_entry_controls_clearbit(vmx, VM_ENTRY_LOAD_IA32_PKRS);
> + vm_exit_controls_clearbit(vmx, VM_EXIT_LOAD_IA32_PKRS);
> + }
> }
