Re: [PATCH 1/2] KVM: KVM-on-hyperv: shorten no-entry section on reenlightenment

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Aug 11, 2021 at 06:23:55AM -0400, Paolo Bonzini wrote:
> During re-enlightenment, update kvmclock a VM at a time instead of
> raising KVM_REQ_MASTERCLOCK_UPDATE for all VMs.  Because the guests
> can now run after TSC emulation has been disabled, invalidate
> their TSC page so that they refer to the reference time counter
> MSR while the update is in progress.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/x86.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bab8eb3e0a47..284afaa1db86 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8111,7 +8111,7 @@ static void kvm_hyperv_tsc_notifier(void)
>  
>  	mutex_lock(&kvm_lock);
>  	list_for_each_entry(kvm, &vm_list, vm_list)
> -		kvm_make_mclock_inprogress_request(kvm);
> +		kvm_hv_invalidate_tsc_page(kvm);
>  
>  	hyperv_stop_tsc_emulation();

        hyperv_stop_tsc_emulation();

        /* TSC frequency always matches when on Hyper-V */
        for_each_present_cpu(cpu)
                per_cpu(cpu_tsc_khz, cpu) = tsc_khz;
        kvm_max_guest_tsc_khz = tsc_khz; 

Is this safe with a running guest? Why?