linux-kernel - Re: [PATCH] KVM: x86: fix backward migration with async

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Date:   Thu, 1 Feb 2018 14:33:10 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Wanpeng Li <wanpeng.li@...mail.com>, stable@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: fix backward migration with async_PF

On 01/02/2018 14:10, Radim Krčmář wrote:
>>>
>> This check will break migration if the source guest and host both have
>> the recent kernels which support KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT, so
>> I am not sure about it.  Otherwise, the patch is okay!
> Good point, breaking forward migration is worse than doing nothing.
> 
> A compromise solution would be to drop the feature check from the
> hypervisor.  Newer guests would work everywhere and there would be no
> change to old systems, so v4.13-v4.15 guests could at least upgrade.
> 
> Slightly better than doing nothing, IMO,

Yes, I agree.  Just removing the guest_kvm_cpuid_has should be fine.

Paolo