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

[Dr. David Alan Gilbert]

* Vitaly Kuznetsov (vkuznets@redhat.com) wrote:
> "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.

Ouch, that's difficult - the problem with testing this late is that the
migration fails right at the end so it's an unpleasent surprise.

Could you disallow re-enlightenment without tsc-frequency on new machine
types?

Dave

> 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
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK