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

[Eduardo Habkost]

On Thu, Mar 22, 2018 at 04:58:03PM +0300, Roman Kagan wrote:
> On Thu, Mar 22, 2018 at 10:22:18AM -0300, Eduardo Habkost wrote:
> > On Thu, Mar 22, 2018 at 04:00:14PM +0300, Roman Kagan wrote:
> > > On Wed, Mar 21, 2018 at 05:19:24PM -0300, Eduardo Habkost wrote:
> > > > 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 <address@hidden> 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 <address@hidden>
> > > > > > >> ---
> > > > > > >>  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.
> > > 
> > > The problem is that the feature is not fixed by the machine-type, due to
> > > the forgotten property: it only depends on the KVM version.  So, once
> > > (if) we add the property and make the feature deterministic, we'll lose
> > > compatibility one way or another.
> > > 
> > > Or are you suggesting that for pre-2.12 machine types we leave the
> > > property at "decided by your KVM" state?
> > 
> > Yes, that's what I mean.  This looks like the only way to avoid
> > losing features by just cold-rebooting an existing VM.
> > 
> > The scenario I'm thinking is this:
> > 
> > 1) pc-2.11 VM started on host running QEMU 2.11
> > 2) VM migrated to a host containing this patch
> > 3) 1 year later, the VM is shut down and booted again.
> > 4) Things stop working inside the VM because hv-frequency is
> >    unexpectedly gone.
> > 
> > Machine-type compatibility code would avoid (4).
> 
> Right, but (4) typically means that you fail to start your workload from
> a clean state, so you just go and fix it; no data is lost.
> 
> Compare this to a migration to an older KVM which results in your guest
> crashing, where you risk data loss and still have to meddle with
> configs.

True. To make it worse, we are already unable to avoid this crash
on existing VMs without a reboot.  The only case where we can fix
this is if live-migration to older KVM happens after the guest
was rebooted when running on a newer QEMU version.  :(



> 
> > > > 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.
> > > 
> > > What's wrong with requiring it, as we do for all other hv_* properties?
> > 
> > On new machine-types, nothing wrong.
> > 
> > On existing machine-types, see above.
> 
> I wonder if the following can cater to all relevant cases:
> 
> - hv_frequencies property is added, defaulting to "off", so that new
>   users of this feature would need to explicitly turn it on;
> 
> - on pre-2.12 machine types, it's set to the value of hv_time property
>   by the compat code, so that on VMs where this feature could
>   potentially be present it would become required; as a result, these
>   configurations will refuse to start on insufficiently capable KVM,
>   preventing the migration attempts.

This sounds like the safest option.  The cost will be the
inconvenience of being unable to run pc-2.11 on hosts with older
KVM (Linux < v4.14, without commit
72c139bacfa386145d7bbb68c47c8824716153b6), and the need to
explicitly enable hv-frequencies on pc-2.12 and newer.

> 
> Am I missing any scenarios that aren't covered?
> 

It looks like the guest can still crash if we migrate
"QEMU-2.12 -machine pc-2.11 -cpu ...,+hv-time" to a host running
QEMU 2.11 and Linux < 4.14.  I wonder if there's a way to avoid
that?  If there's a way to avoid that with extra migration
subsections, is it worth the effort/complexity?

-- 
Eduardo