Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Mar 21, 2018 at 07:57:29PM +0300, Roman Kagan wrote:
> On Wed, Mar 21, 2018 at 02:18:54PM +0100, Vitaly Kuznetsov wrote:
> > Roman Kagan <rkagan@virtuozzo.com> writes:
> > 
> > > On Tue, Mar 20, 2018 at 06:35:00PM +0100, Vitaly Kuznetsov wrote:
> > >> Requiring tsc_is_stable_and_known() is too restrictive: even without INVTCS
> > >> nested Hyper-V-on-KVM enables TSC pages for its guests e.g. when
> > >> Reenlightenment MSRs are present. Presence of frequency MSRs doesn't mean
> > >> these frequencies are stable, it just means they're available for reading.
> > >> 
> > >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> > >> ---
> > >>  target/i386/kvm.c | 2 +-
> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >> 
> > >> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> > >> index 7d9f9ca0b1..74fc3d3b2c 100644
> > >> --- a/target/i386/kvm.c
> > >> +++ b/target/i386/kvm.c
> > >> @@ -651,7 +651,7 @@ static int hyperv_handle_properties(CPUState *cs)
> > >>          env->features[FEAT_HYPERV_EAX] |= HV_TIME_REF_COUNT_AVAILABLE;
> > >>          env->features[FEAT_HYPERV_EAX] |= HV_REFERENCE_TSC_AVAILABLE;
> > >>  
> > >> -        if (has_msr_hv_frequencies && tsc_is_stable_and_known(env)) {
> > >> +        if (has_msr_hv_frequencies && env->tsc_khz) {
> > >>              env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
> > >>              env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
> > >>          }
> > >
> > > I suggest that we add a corresponding cpu property here, too.  The guest
> > > may legitimately rely on these msrs when it sees the support in CPUID,
> > > and migrating from a kernel with the feature supported (4.14+) to an
> > > older one will make it crash.
> > >
> > 
> > This can be arranged, but what happens to people who use these features
> > today? Assuming they also passed 'invtsc' they have stable TSC page
> > clocksource already (when Hyper-V role is enabled) but when we start
> > requesting a new 'hv_frequency' cpu property they'll suddenly lose what
> > they have...
> 
> I see two cases here:
> 
> 1) people start a new VM, and discover that their old configuration is
>    not enough for this feature to work.
> 
>    They need to reconfigure and restart the VM.  This costs them some
>    time investigating and restarting, but not data.

If we keep machine-type compatibility, people will need to do
that only if they change the machine-type (or use the "pc" or
"q35" aliases).  If they copy the old configuration, it will keep
working.

machine-type compatibility also makes the following case a bit
safer:

> 
> 2) people migrate from a QEMU without ->hv_frequency, to a new one with
>    ->hv_frequency=off (assuming on both ends KVM supports the frequency
>    MSRs).
> 
>    With the current implementation in KVM, this will only result in the
>    feature bits disappearing from the respective CPUID leaf, but the
>    MSRs themselves will continue working as they used to.  So the guest
>    either won't notice or will check the CPUID and adjust.

If we keep machine-type compatibility, the CPUID bit won't
disappear for the guest while the MSRs keep working.


Whichever solution we choose, we can still have guests crashing
if migrating a pc-2.11 machine from a 4.14+ host kernel to a host
with an older kernel.  But I don't think there's a way out of
this, except requiring an explicit "hv-frequencies" CPU option on
newer machine-types.

-- 
Eduardo