Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is trustworthy

Jim Mattson
2019-11-05T17:35:27Z

On Tue, Nov 5, 2019 at 9:32 AM Liran Alon <liran.alon@oracle.com> wrote:
>
>
>
> > On 5 Nov 2019, at 19:17, Liran Alon <liran.alon@oracle.com> wrote:
> >
> >
> >
> >> On 5 Nov 2019, at 18:17, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> >>
> >> Virtualized guests may pick a different strategy to mitigate hardware
> >> vulnerabilities when it comes to hyper-threading: disable SMT completely,
> >> use core scheduling, or, for example, opt in for STIBP. Making the
> >> decision, however, requires an extra bit of information which is currently
> >> missing: does the topology the guest see match hardware or if it is 'fake'
> >> and two vCPUs which look like different cores from guest's perspective can
> >> actually be scheduled on the same physical core. Disabling SMT or doing
> >> core scheduling only makes sense when the topology is trustworthy.
> >
> > This is not only related to vulnerability mitigations.
> > It’s also important for guest to know if it’s SMT topology is trustworthy for various optimisation algorithms.
> > E.g. Should it attempt to run tasks that share memory on same NUMA node?
> >
> >>
> >> Add two feature bits to KVM: KVM_FEATURE_TRUSTWORTHY_SMT with the meaning
> >> that KVM_HINTS_TRUSTWORTHY_SMT bit answers the question if the exposed SMT
> >> topology is actually trustworthy. It would, of course, be possible to get
> >> away with a single bit (e.g. 'KVM_FEATURE_FAKE_SMT') and not lose backwards
> >> compatibility but the current approach looks more straightforward.
> >
> > Agree.
> >
> >>
> >> There were some offline discussions on whether this new feature bit should
> >> be complemented with a 're-enlightenment' mechanism for live migration (so
> >> it can change in guest's lifetime) but it doesn't seem to be very
> >> practical: what a sane guest is supposed to do if it's told that SMT
> >> topology is about to become fake other than kill itself? Also, it seems to
> >> make little sense to do e.g. CPU pinning on the source but not on the
> >> destination.
> >
> > Agree.
> >
> >>
> >> There is also one additional piece of the information missing. A VM can be
> >> sharing physical cores with other VMs (or other userspace tasks on the
> >> host) so does KVM_FEATURE_TRUSTWORTHY_SMT imply that it's not the case or
> >> not? It is unclear if this changes anything and can probably be left out
> >> of scope (just don't do that).
> >
> > I don’t think KVM_FEATURE_TRUSTWORTHY_SMT should indicate to guest whether it’s vCPU shares a CPU core with another guest.
> > It should only expose to guest the fact that he can rely on it’s virtual SMT topology. i.e. That there is a relation between virtual SMT topology
> > to which physical logical processors run which vCPUs.
> >
> > Guest have nothing to do with the fact that he is now aware host doesn’t guarantee to him that one of it’s vCPU shares a CPU core with another guest vCPU.
> > I don’t think we should have a CPUID bit that expose this information to guest.
> >
> >>
> >> Similar to the already existent 'NoNonArchitecturalCoreSharing' Hyper-V
> >> enlightenment, the default value of KVM_HINTS_TRUSTWORTHY_SMT is set to
> >> !cpu_smt_possible(). KVM userspace is thus supposed to pass it to guest's
> >> CPUIDs in case it is '1' (meaning no SMT on the host at all) or do some
> >> extra work (like CPU pinning and exposing the correct topology) before
> >> passing '1' to the guest.
> >
> > Hmm… I’m not sure this is correct.
> > For example, it is possible to expose in virtual SMT topology that guest have 2 vCPUs running on single NUMA node,
> > while in reality each vCPU task can be scheduled to run on different NUMA nodes. Therefore, making virtual SMT topology not trustworthy.
> > i.e. Disabling SMT on host doesn’t mean that virtual SMT topology is reliable.
> >
> > I think this CPUID bit should just be set from userspace when admin have guaranteed to guest that it have set vCPU task affinity properly.
> > Without KVM attempting to set this bit by itself.
> >
> > Note that we defined above KVM_HINTS_TRUSTWORTHY_SMT bit differently than “NoNonArchitecturalCoreSharing”.
> > “NoNonArchitecturalCoreSharing” guarantees to guest that vCPUs of guest won’t share a physical CPU core unless they are defined as virtual SMT siblings.
> > In contrast, KVM_HINTS_TRUSTWORTHY_SMT bit attempts to state that virtual SMT topology is a subset of how vCPUs are scheduled on physical SMT topology.
> > i.e. It seems that Hyper-V bit is indeed only attempting to provide guest information related to security mitigations. While newly proposed KVM bit attempts to also
> > assist guest to determine how to perform it’s internal scheduling decisions.
> >
> > -Liran
>
> Oh I later saw below that you defined KVM_HINTS_TRUSTWORTHY_SMT indeed as Microsoft defined “NoNonArchitecturalCoreSharing”.
> If you plan to go with this direction, than I suggest renaming to similar name as Hyper-V.
> But I think having a general vSMT topology is trustworthy is also useful.
> Maybe we should have separate bits for each.

And perhaps a bit each for "vCCX topology is trustworthy" and "vNUMA
topology is trustworthy"?

> -Liran
>
> >
> >>
> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> >> ---
> >> Documentation/virt/kvm/cpuid.rst | 27 +++++++++++++++++++--------
> >> arch/x86/include/uapi/asm/kvm_para.h | 2 ++
> >> arch/x86/kvm/cpuid.c | 7 ++++++-
> >> 3 files changed, 27 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/Documentation/virt/kvm/cpuid.rst b/Documentation/virt/kvm/cpuid.rst
> >> index 01b081f6e7ea..64b94103fc90 100644
> >> --- a/Documentation/virt/kvm/cpuid.rst
> >> +++ b/Documentation/virt/kvm/cpuid.rst
> >> @@ -86,6 +86,10 @@ KVM_FEATURE_PV_SCHED_YIELD 13 guest checks this feature bit
> >> before using paravirtualized
> >> sched yield.
> >>
> >> +KVM_FEATURE_TRUSTWORTHY_SMT 14 set when host supports 'SMT
> >> + topology is trustworthy' hint
> >> + (KVM_HINTS_TRUSTWORTHY_SMT).
> >> +
> >> KVM_FEATURE_CLOCSOURCE_STABLE_BIT 24 host will warn if no guest-side
> >> per-cpu warps are expeced in
> >> kvmclock
> >> @@ -97,11 +101,18 @@ KVM_FEATURE_CLOCSOURCE_STABLE_BIT 24 host will warn if no guest-side
> >>
> >> Where ``flag`` here is defined as below:
> >>
> >> -================== ============ =================================
> >> -flag value meaning
> >> -================== ============ =================================
> >> -KVM_HINTS_REALTIME 0 guest checks this feature bit to
> >> - determine that vCPUs are never
> >> - preempted for an unlimited time
> >> - allowing optimizations
> >> -================== ============ =================================
> >> +================================= =========== =================================
> >> +flag value meaning
> >> +================================= =========== =================================
> >> +KVM_HINTS_REALTIME 0 guest checks this feature bit to
> >> + determine that vCPUs are never
> >> + preempted for an unlimited time
> >> + allowing optimizations
> >> +
> >> +KVM_HINTS_TRUSTWORTHY_SMT 1 the bit is set when the exposed
> >> + SMT topology is trustworthy, this
> >> + means that two guest vCPUs will
> >> + never share a physical core
> >> + unless they are exposed as SMT
> >> + threads.
> >> +================================= =========== =================================
> >> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> >> index 2a8e0b6b9805..183239d5dfad 100644
> >> --- a/arch/x86/include/uapi/asm/kvm_para.h
> >> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> >> @@ -31,8 +31,10 @@
> >> #define KVM_FEATURE_PV_SEND_IPI 11
> >> #define KVM_FEATURE_POLL_CONTROL 12
> >> #define KVM_FEATURE_PV_SCHED_YIELD 13
> >> +#define KVM_FEATURE_TRUSTWORTHY_SMT 14
> >>
> >> #define KVM_HINTS_REALTIME 0
> >> +#define KVM_HINTS_TRUSTWORTHY_SMT 1
> >>
> >> /* The last 8 bits are used to indicate how to interpret the flags field
> >> * in pvclock structure. If no bits are set, all flags are ignored.
> >> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> >> index f68c0c753c38..dab527a7081f 100644
> >> --- a/arch/x86/kvm/cpuid.c
> >> +++ b/arch/x86/kvm/cpuid.c
> >> @@ -712,7 +712,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >> (1 << KVM_FEATURE_ASYNC_PF_VMEXIT) |
> >> (1 << KVM_FEATURE_PV_SEND_IPI) |
> >> (1 << KVM_FEATURE_POLL_CONTROL) |
> >> - (1 << KVM_FEATURE_PV_SCHED_YIELD);
> >> + (1 << KVM_FEATURE_PV_SCHED_YIELD) |
> >> + (1 << KVM_FEATURE_TRUSTWORTHY_SMT);
> >>
> >> if (sched_info_on())
> >> entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
> >> @@ -720,6 +721,10 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >> entry->ebx = 0;
> >> entry->ecx = 0;
> >> entry->edx = 0;
> >> +
> >> + if (!cpu_smt_possible())
> >> + entry->edx |= (1 << KVM_HINTS_TRUSTWORTHY_SMT);
> >> +
> >> break;
> >> case 0x80000000:
> >> entry->eax = min(entry->eax, 0x8000001f);
> >> --
> >> 2.20.1
> >>
> >
>