Message-ID: <61325b2b-dc93-5db2-2d0a-dd0900d947f2@redhat.com>
Date:   Sun, 2 Jan 2022 18:31:50 +0100
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Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN
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To:     Igor Mammedov <imammedo@redhat.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>, kvm@vger.kernel.org,
        Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>, linux-kernel@vger.kernel.org
References: <20211122175818.608220-1-vkuznets@redhat.com>
 <20211122175818.608220-3-vkuznets@redhat.com>
 <16368a89-99ea-e52c-47b6-bd006933ec1f@redhat.com>
 <20211227183253.45a03ca2@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20211227183253.45a03ca2@redhat.com>
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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