Re: [PATCH v2] i386: Make migration fail when Hyper-V reenlightenment was enabled but 'user_tsc_khz' is unset

[Vitaly Kuznetsov <vkuznets@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:

> * Vitaly Kuznetsov (vkuznets@redhat.com) wrote:
>> KVM doesn't fully support Hyper-V reenlightenment notifications on
>> migration. In particular, it doesn't support emulating TSC frequency
>> of the source host by trapping all TSC accesses so unless TSC scaling
>> is supported on the destination host and KVM_SET_TSC_KHZ succeeds, it
>> is unsafe to proceed with migration.
>> 
>> KVM_SET_TSC_KHZ is called from two sites: kvm_arch_init_vcpu() and
>> kvm_arch_put_registers(). The later (intentionally) doesn't propagate
>> errors allowing migrations to succeed even when TSC scaling is not
>> supported on the destination. This doesn't suit 're-enlightenment'
>> use-case as we have to guarantee that TSC frequency stays constant.
>> 
>> Require 'tsc-frequency=' command line option to be specified for successful
>> migration when re-enlightenment was enabled by the guest.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>> This patch is a successor of "[PATCH 3/3] i386: Make sure
>> kvm_arch_set_tsc_khz() succeeds on migration when 'hv-reenlightenment'
>> was exposed" taking a different approach suggested by Paolo.
>> ---
>>  docs/hyperv.txt                |  5 +++++
>>  target/i386/kvm/hyperv-proto.h |  1 +
>>  target/i386/machine.c          | 20 ++++++++++++++++++++
>>  3 files changed, 26 insertions(+)
>> 
>> diff --git a/docs/hyperv.txt b/docs/hyperv.txt
>> index 5df00da54fc4..e53c581f4586 100644
>> --- a/docs/hyperv.txt
>> +++ b/docs/hyperv.txt
>> @@ -160,6 +160,11 @@ the hypervisor) until it is ready to switch to the new one. This, in conjunction
>>  with hv-frequencies, allows Hyper-V on KVM to pass stable clocksource (Reference
>>  TSC page) to its own guests.
>>  
>> +Note, KVM doesn't fully support re-enlightenment notifications and doesn't
>> +emulate TSC accesses after migration so 'tsc-frequency=' CPU option also has to
>> +be specified to make migration succeed. The destination host has to either have
>> +the same TSC frequency or support TSC scaling CPU feature.
>> +
>>  Recommended: hv-frequencies
>>  
>>  3.16. hv-evmcs
>> diff --git a/target/i386/kvm/hyperv-proto.h b/target/i386/kvm/hyperv-proto.h
>> index 056a305be38c..e30d64b4ade4 100644
>> --- a/target/i386/kvm/hyperv-proto.h
>> +++ b/target/i386/kvm/hyperv-proto.h
>> @@ -139,6 +139,7 @@
>>   * Reenlightenment notification MSRs
>>   */
>>  #define HV_X64_MSR_REENLIGHTENMENT_CONTROL      0x40000106
>> +#define HV_REENLIGHTENMENT_ENABLE_BIT           (1u << 16)
>>  #define HV_X64_MSR_TSC_EMULATION_CONTROL        0x40000107
>>  #define HV_X64_MSR_TSC_EMULATION_STATUS         0x40000108
>>  
>> diff --git a/target/i386/machine.c b/target/i386/machine.c
>> index 7259fe6868c6..137604ddb898 100644
>> --- a/target/i386/machine.c
>> +++ b/target/i386/machine.c
>> @@ -883,11 +883,31 @@ static bool hyperv_reenlightenment_enable_needed(void *opaque)
>>          env->msr_hv_tsc_emulation_status != 0;
>>  }
>>  
>> +static int hyperv_reenlightenment_post_load(void *opaque, int version_id)
>> +{
>> +    X86CPU *cpu = opaque;
>> +    CPUX86State *env = &cpu->env;
>> +
>> +    /*
>> +     * KVM doesn't fully support re-enlightenment notifications so we need to
>> +     * make sure TSC frequency doesn't change upon migration.
>> +     */
>> +    if ((env->msr_hv_reenlightenment_control & HV_REENLIGHTENMENT_ENABLE_BIT) &&
>> +        !env->user_tsc_khz) {
>> +        error_report("Guest enabled re-enlightenment notifications, "
>> +                     "'tsc-frequency=' has to be specified");
>
> It's unusual to fail on the destination for a valid configuration but
> guest state;  wouldn't it be better to always insist on tsc-frequency if
> that hv feature is exposed; failing early before reeiving the state?
>

Doing so would make a number of currently existing configurations
invalid, even when re-enlightenment is not to be used by the
guest. AFAIR Windows without Hyper-V doesn't enable it. Generally, we
just advise people to 'enable all currently supported hyper-v
enlightenments' to make things easier so reenlightenment may end up
being added for no particular reason.

FWIW, v1 of this patch used a slightly different approach: I was
requiring kvm_arch_set_tsc_khz() to succeed. This is not exactly the
same as e.g. even when the destination host doesn't support TSC scaling,
it may happen to have the same TSC frequency.

-- 
Vitaly