linux-kernel - Re: [PATCH 6/6] x86/kvm: support Hyper-V reenlightenment

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Date:   Fri, 8 Dec 2017 20:39:10 +0300
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Andy Lutomirski <luto@...nel.org>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, linux-kernel@...r.kernel.org,
        devel@...uxdriverproject.org
Subject: Re: [PATCH 6/6] x86/kvm: support Hyper-V reenlightenment

On Fri, Dec 08, 2017 at 11:50:00AM +0100, Vitaly Kuznetsov wrote:
> When we run nested KVM on Hyper-V guests we need to update masterclocks for
> all guests when L1 migrates to a host with different TSC frequency.
> Implement the procedure in the following way:
> - Pause all guests.
> - Tell our host (Hyper-V) to stop emulating TSC accesses.
> - Update our gtod copy, recompute clocks.
> - Unpause all guests.
> 
> This is somewhat similar to cpufreq but we have two important differences:
> we can only disable TSC emulation globally (on all CPUs) and we don't know
> the new TSC frequency until we turn the emulation off so we can't
> 'prepare' ourselves to the event.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 96e04a0cb921..04d90712ffd2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -68,6 +68,7 @@
>  #include <asm/div64.h>
>  #include <asm/irq_remapping.h>
>  #include <asm/mshyperv.h>
> +#include <asm/hypervisor.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include "trace.h"
> @@ -5946,6 +5947,43 @@ static void tsc_khz_changed(void *data)
>  	__this_cpu_write(cpu_tsc_khz, khz);
>  }
>  
> +void kvm_hyperv_tsc_notifier(void)
> +{
> +#ifdef CONFIG_X86_64
> +	struct kvm *kvm;
> +	struct kvm_vcpu *vcpu;
> +	int cpu;
> +
> +	spin_lock(&kvm_lock);
> +	list_for_each_entry(kvm, &vm_list, vm_list)
> +		kvm_make_mclock_inprogress_request(kvm);
> +
> +	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;
> +
> +	list_for_each_entry(kvm, &vm_list, vm_list) {
> +		struct kvm_arch *ka = &kvm->arch;
> +
> +		spin_lock(&ka->pvclock_gtod_sync_lock);
> +
> +		pvclock_update_vm_gtod_copy(kvm);
> +
> +		kvm_for_each_vcpu(cpu, vcpu, kvm)
> +			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> +
> +		kvm_for_each_vcpu(cpu, vcpu, kvm)
> +			kvm_clear_request(KVM_REQ_MCLOCK_INPROGRESS, vcpu);
> +
> +		spin_unlock(&ka->pvclock_gtod_sync_lock);
> +	}
> +	spin_unlock(&kvm_lock);

Can't you skip all this if the tsc frequency hasn't changed (which
should probably be the case when the CPU supports tsc frequency
scaling)?

Roman.