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

[Ankur Arora]

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.


Ankur

> Paolo
6