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Subject: Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is
 trustworthy
To:     Paolo Bonzini <pbonzini@redhat.com>,
        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>
 <de3cade3-c069-dc6b-1d2d-aa10abe365b8@redhat.com>
From:   Ankur Arora <ankur.a.arora@oracle.com>
Message-ID: <4f835a11-1528-a04e-9e06-1b8cdb97a04d@oracle.com>
Date:   Thu, 5 Dec 2019 20:01:13 -0800
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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
> 
6