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 <[email protected]> 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 <[email protected]> > > >> --- > > >> 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
