From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Igor Mammedov <imammedo@redhat.com>
Cc:     kvm@vger.kernel.org, Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN
In-Reply-To: <87o84qpk7d.fsf@redhat.com>
References: <20211122175818.608220-1-vkuznets@redhat.com>
 <20211122175818.608220-3-vkuznets@redhat.com>
 <16368a89-99ea-e52c-47b6-bd006933ec1f@redhat.com>
 <20211227183253.45a03ca2@redhat.com>
 <61325b2b-dc93-5db2-2d0a-dd0900d947f2@redhat.com>
 <87mtkdqm7m.fsf@redhat.com> <20220103104057.4dcf7948@redhat.com>
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Date:   Fri, 07 Jan 2022 10:02:49 +0100
Message-ID: <877dbbq5om.fsf@redhat.com>
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Vitaly Kuznetsov <vkuznets@redhat.com> writes:

> Paolo Bonzini <pbonzini@redhat.com> writes:
>
>> On 1/3/22 13:56, Vitaly Kuznetsov wrote:
>>>   'allowlist' of things which can change (and put
>>> *APICids there) and only fail KVM_SET_CPUID{,2} when we see something
>>> else changing.
>>
>> We could also go the other way and only deny changes that result in 
>> changed CPU caps.  That should be easier to implement since we have 
>> already a mapping from CPU capability words to CPUID leaves and registers.
>>
>
> Good idea, I'll look into it (if noone beats me to it).

(just getting to it)

On the other hand, e.g. MAXPHYADDR (CPUID 0x80000008.EAX) is not a CPU
cap but it's one of the main reasons why we want to forbid
KVM_SET_CPUID{,2} after KVM_RUN in the first place. I'm also not sure
about allowing PV feature bits changes (KVM, Hyper-V, Xen) and I don't
think there's a need for that.

I'm again leaning towards an allowlist and currently I see only two
candidates:

CPUID.01H.EBX bits 31:24 (initial LAPIC id)
CPUID.0BH.EDX (x2APIC id)

Anything else I'm missing?

-- 
Vitaly