linux-kernel - Re: [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM

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Date:   Tue, 18 Jan 2022 17:17:01 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after
 KVM_RUN for CPU hotplug

On 1/18/22 15:35, Igor Mammedov wrote:
> Can you check following scenario:
>   * on host that has IA32_TSX_CTRL and TSX enabled (RTM/HLE cpuid bits present)
>   * boot 2 vcpus VM with TSX enabled on VMM side but with tsx=off on kernel CLI
> 
>       that should cause kernel to set MSR_IA32_TSX_CTRL to 3H from initial 0H
>       and clear RTM+HLE bits in CPUID, check that RTM/HLE cpuid it cleared
> 
>   * hotunplug a VCPU and then replug it again
>      if IA32_TSX_CTRL is reset to initial state, that should re-enable
>      RTM/HLE cpuid bits and KVM_SET_CPUID2 might fail due to difference
> 
> and as Sean pointed out there might be other non constant leafs,
> where exact match check could leave userspace broken.


MSR_IA32_TSX_CTRL is handled differently straight during the CPUID call:

                 if (function == 7 && index == 0) {
                         u64 data;
                         if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
                             (data & TSX_CTRL_CPUID_CLEAR))
                                 *ebx &= ~(F(RTM) | F(HLE));
                 }


and I think we should redo all or most of kvm_update_cpuid_runtime
the same way.

Paolo