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Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V
From: |
Eduardo Habkost |
Subject: |
Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure |
Date: |
Thu, 22 Mar 2018 10:22:18 -0300 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
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).
>
> >
> > 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.
--
Eduardo
- Re: [Qemu-devel] [PATCH v3 1/2] i386/kvm: add support for Hyper-V reenlightenment MSRs, (continued)
- [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Vitaly Kuznetsov, 2018/03/20
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/21
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Vitaly Kuznetsov, 2018/03/21
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/21
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Eduardo Habkost, 2018/03/21
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/22
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure,
Eduardo Habkost <=
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/22
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Eduardo Habkost, 2018/03/22
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/23
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Eduardo Habkost, 2018/03/23
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/26
Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Paolo Bonzini, 2018/03/21
Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/21