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

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Message-ID: <87mtkdqm7m.fsf@redhat.com>
Date:   Mon, 03 Jan 2022 09:04:29 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Igor Mammedov <imammedo@...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 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

Paolo Bonzini <pbonzini@...hat.com> writes:

> 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.

Are we sure the data is going to be *exactly* the same? In particular,
when using vCPU fds from the parked list, do we keep the same
APIC/x2APIC id when hotplugging? Or can we actually hotplug with a
different id?

-- 
Vitaly