"Dr. David Alan Gilbert" <dgilb...@redhat.com> writes:
> * Vitaly Kuznetsov (vkuzn...@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 <vkuzn...@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
