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

[Vitaly Kuznetsov]

Ankur Arora <ankur.a.arora@xxxxxxxxxx> writes:

> On 2019-11-05 3:56 p.m., Paolo Bonzini wrote:
>> On 05/11/19 17:17, Vitaly Kuznetsov wrote:
>>> 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).
>>>
>>> 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.
>>>
>>> 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).
>>> +
>> 
>> Instead of defining a one-off bit, can we make:
>> 
>> ecx = the set of known "hints" (defaults to edx if zero)
>> 
>> edx = the set of hints that apply to the virtual machine
>>
> Just to resurrect this thread, the guest could explicitly ACK
> a KVM_FEATURE_DYNAMIC_HINT at init. This would allow the host
> to change the hints whenever with the guest not needing to separately
> ACK the changed hints.

(I apologize for dropping the ball on this, I'm intended to do RFCv2 in
a nearby future)

Regarding this particular hint (let's call it 'no nonarchitectural
coresharing' for now) I don't see much value in communicating change to
guest when it happens. Imagine our host for some reason is not able to
guarantee that anymore e.g. we've migrated to a host with less pCPUs
and/or special restrictions and have to start sharing. What we, as a
guest, are supposed to do when we receive a notification? "You're now
insecure, deal with it!" :-) Equally, I don't see much value in
pre-acking such change. "I'm fine with becoming insecure at some point".

If we, however, discuss other hints such 'pre-ACK' mechanism may make
sense, however, I'd make it an option to a 'challenge/response'
protocol: if host wants to change a hint it notifies the guest and waits
for an ACK from it (e.g. a pair of MSRs + an interrupt). I, however,
have no good candidate from the existing hints which would require guest
to ACK (e.g revoking PV EOI would probably do but why would we do that?)
As I said before, challenge/response protocol is needed if we'd like to
make TSC frequency change the way Hyper-V does it (required for updating
guest TSC pages in nested case) but this is less and less important with
the appearance of TSC scaling. I'm still not sure if this is an
over-engineering or not. We can wait for the first good candidate to
decide.

-- 
Vitaly