Re: [PATCH] X86/KVM: Do not allow DISABLE_EXITS_MWAIT when LAPIC ARAT is not available

[Sean Christopherson]

On Wed, 2018-04-11 at 11:16 +0200, KarimAllah Ahmed wrote:
> If the processor does not have an "Always Running APIC Timer" (aka ARAT),
> we should not give guests direct access to MWAIT. The LAPIC timer would
> stop ticking in deep C-states, so any host deadlines would not wakeup the
> host kernel.
> 
> The host kernel intel_idle driver handles this by switching to broadcast
> mode when ARAT is not available and MWAIT is issued with a deep C-state
> that would stop the LAPIC timer. When MWAIT is passed through, we can not
> tell when MWAIT is issued.
> 
> So just disable this capability when LAPIC ARAT is not available. I am not
> even sure if there are any CPUs with VMX support but no LAPIC ARAT or not.

ARAT was added on WSM, so NHM, the Core 2 family and a few PSC SKUs
support VMX+MWAIT but not ARAT. Â

> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Reported-by: Wanpeng Li <kernellwp@xxxxxxxxx>
> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> ---
> Âarch/x86/kvm/x86.c | 3 ++-
> Â1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b2ff74b..0334b25 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2819,7 +2819,8 @@ static int msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs __user *user_msrs,
> Âstatic inline bool kvm_can_mwait_in_guest(void)
> Â{
> Â	return boot_cpu_has(X86_FEATURE_MWAIT) &&
> -		!boot_cpu_has_bug(X86_BUG_MONITOR);
> +		!boot_cpu_has_bug(X86_BUG_MONITOR) &&
> +		boot_cpu_has(X86_FEATURE_ARAT);
> Â}
> Â
> Âint kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)