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

[Liran Alon]

> On 5 Nov 2019, at 19:17, Liran Alon <liran.alon@xxxxxxxxxx> wrote:
> 
> 
> 
>> On 5 Nov 2019, at 18:17, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> 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.
> Itâs also important for guest to know if itâs 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 donât think KVM_FEATURE_TRUSTWORTHY_SMT should indicate to guest whether itâs vCPU shares a CPU core with another guest.
> It should only expose to guest the fact that he can rely on itâs 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 doesnât guarantee to him that one of itâs vCPU shares a CPU core with another guest vCPU.
> I donât 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.
> 
> Hmmâ Iâm 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 doesnât 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 âNoNonArchitecturalCoreSharingâ.
> âNoNonArchitecturalCoreSharingâ guarantees to guest that vCPUs of guest wonât 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 itâs internal scheduling decisions.
> 
> -Liran

Oh I later saw below that you defined KVM_HINTS_TRUSTWORTHY_SMT indeed as Microsoft defined âNoNonArchitecturalCoreSharingâ.
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.

-Liran

> 
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> ---
>> 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
>> 
>