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Subject: Re: [PATCH] KVM: x86: fix backward migration with async_PF
To:     =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Wanpeng Li <wanpeng.li@hotmail.com>, stable@vger.kernel.org
References: <20180201175022.25011-1-rkrcmar@redhat.com>
 <a4fa9a87-cc4e-73d6-e248-192ab167b2c2@redhat.com>
 <20180201191040.GD26932@flask>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <c820ef8e-b352-ef28-4756-d6949f718ec0@redhat.com>
Date:   Thu, 1 Feb 2018 14:33:10 -0500
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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