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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Dexuan Cui <decui@microsoft.com>
Cc:     bp@alien8.de, haiyangz@microsoft.com, hpa@zytor.com,
        kys@microsoft.com, linux-hyperv@vger.kernel.org,
        linux-kernel@vger.kernel.org, mingo@redhat.com,
        sthemmin@microsoft.com, tglx@linutronix.de, x86@kernel.org,
        mikelley@microsoft.com, wei.liu@kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH] x86/hyperv: Suspend/resume the VP assist page for hibernation
In-Reply-To: <1587104999-28927-1-git-send-email-decui@microsoft.com>
References: <1587104999-28927-1-git-send-email-decui@microsoft.com>
Date:   Fri, 17 Apr 2020 12:03:18 +0200
Message-ID: <87blnqv389.fsf@vitty.brq.redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Dexuan Cui <decui@microsoft.com> writes:

> Unlike the other CPUs, CPU0 is never offlined during hibernation. So in the
> resume path, the "new" kernel's VP assist page is not suspended (i.e.
> disabled), and later when we jump to the "old" kernel, the page is not
> properly re-enabled for CPU0 with the allocated page from the old kernel.
>
> So far, the VP assist page is only used by hv_apic_eoi_write().

No, not only for that ('git grep hv_get_vp_assist_page')

KVM on Hyper-V also needs VP assist page to use Enlightened VMCS. In
particular, Enlightened VMPTR is written there.

This makes me wonder: how does hibernation work with KVM in case we use
Enlightened VMCS and we have VMs running? We need to make sure VP Assist
page content is preserved.

-- 
Vitaly