linux-kernel - Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM

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Date:   Sun, 2 Jan 2022 18:31:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Igor Mammedov <imammedo@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, 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 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

On 12/27/21 18:32, Igor Mammedov wrote:
>> Tweaked and queued nevertheless, thanks.
> it seems this patch breaks VCPU hotplug, in scenario:
> 
>    1. hotunplug existing VCPU (QEMU stores VCPU file descriptor in parked cpus list)
>    2. hotplug it again (unsuspecting QEMU reuses stored file descriptor when recreating VCPU)
> 
> RHBZ:https://bugzilla.redhat.com/show_bug.cgi?id=2028337#c11
> 

The fix here would be (in QEMU) to not call KVM_SET_CPUID2 again. 
However, we need to work around it in KVM, and allow KVM_SET_CPUID2 if 
the data passed to the ioctl is the same that was set before.

Paolo