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Subject: Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is
 trustworthy
To:     Vitaly Kuznetsov <vkuznets@redhat.com>, kvm@vger.kernel.org
Cc:     x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Jim Mattson <jmattson@google.com>,
        Liran Alon <liran.alon@oracle.com>,
        linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
References: <20191105161737.21395-1-vkuznets@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Wed, 6 Nov 2019 00:56:15 +0100
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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

Paolo