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

Ankur Arora
2019-12-06T04:02:03Z

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@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).
>> +
>
> 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.

Ankur

>
> Paolo
>
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