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SubjectRe: [PATCH v2 15/28] KVM: VMX: Check CPU_BASED_{INTR,NMI}_WINDOW_EXITING in setup_vmcs_config()
On Wed, Jun 29, 2022, Vitaly Kuznetsov wrote:
> CPU_BASED_{INTR,NMI}_WINDOW_EXITING controls are toggled dynamically by
> vmx_enable_{irq,nmi}_window, handle_interrupt_window(), handle_nmi_window()
> but setup_vmcs_config() doesn't check their existence. Add the check and
> filter the controls out in vmx_exec_control().
>
> No (real) functional change intended as all existing CPUs supporting
> VMX are supposed to have these controls.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index da8bbba38d0e..89a3bbafa5af 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2487,7 +2487,9 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
> CPU_BASED_MWAIT_EXITING |
> CPU_BASED_MONITOR_EXITING |
> CPU_BASED_INVLPG_EXITING |
> - CPU_BASED_RDPMC_EXITING;
> + CPU_BASED_RDPMC_EXITING |
> + CPU_BASED_INTR_WINDOW_EXITING |
> + CPU_BASED_NMI_WINDOW_EXITING;

Requiring NMI_WINDOW_EXITING is wrong, KVM doesn't use NMI_WINDOW_EXITING if
enable_vnmi == false:

if (!enable_vnmi ||
vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) & GUEST_INTR_STATE_STI) {
vmx_enable_irq_window(vcpu);
return;
}

exec_controls_setbit(to_vmx(vcpu), CPU_BASED_NMI_WINDOW_EXITING);

And enable_vnmi is cleared if PIN_BASED_VIRTUAL_NMIS are unsupported:

if (!cpu_has_virtual_nmis())
enable_vnmi = 0;

I suspect CPUs that don't support VIRTUAL_NMIS also don't support NMI_WINDOW_EXITING,
and KVM explicitly supports such CPUs. See commit 8a1b43922d0d ("kvm: vmx: Reinstate
support for CPUs without virtual NMI").

Making NMI_WINDOW_EXITING optional and then adding it to cpu_has_virtual_nmis()
seems like the correct approach.

diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index 069d8d298e1d..c34890e9fc2a 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -82,7 +82,8 @@ static inline bool cpu_has_vmx_basic_inout(void)

static inline bool cpu_has_virtual_nmis(void)
{
- return vmcs_config.pin_based_exec_ctrl & PIN_BASED_VIRTUAL_NMIS;
+ return vmcs_config.pin_based_exec_ctrl & PIN_BASED_VIRTUAL_NMIS &&
+ vmcs_config.cpu_based_exec_ctrl & CPU_BASED_NMI_WINDOW_EXITING;
}

static inline bool cpu_has_vmx_preemption_timer(void)

> opt = CPU_BASED_TPR_SHADOW |
> CPU_BASED_USE_MSR_BITMAPS |
> @@ -4300,6 +4302,10 @@ static u32 vmx_exec_control(struct vcpu_vmx *vmx)
> {
> u32 exec_control = vmcs_config.cpu_based_exec_ctrl;
>
> + /* INTR_WINDOW_EXITING and NMI_WINDOW_EXITING are toggled dynamically */
> + exec_control &= ~(CPU_BASED_INTR_WINDOW_EXITING |
> + CPU_BASED_NMI_WINDOW_EXITING);
> +
> if (vmx->vcpu.arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)
> exec_control &= ~CPU_BASED_MOV_DR_EXITING;
>
> --
> 2.35.3
>

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