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

[Vitaly Kuznetsov]

Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:

> 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@xxxxxxxxxx>
>> ---
>>  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)?
>

The thing is that we don't know if it changed or not: only after
disabling TSC emulation we'll be able to read the new one from the host
and we need to do this with all VMs paused.

-- 
  Vitaly