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Date:   Fri, 17 Apr 2020 11:55:58 +0100
From:   Wei Liu <wei.liu@kernel.org>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Dexuan Cui <decui@microsoft.com>, 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
Message-ID: <20200417105558.2jkqq2lih6vvoip2@debian>
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 <87blnqv389.fsf@vitty.brq.redhat.com>
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On Fri, Apr 17, 2020 at 12:03:18PM +0200, Vitaly Kuznetsov wrote:
> 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.

The page itself is preserved, isn't it?

hv_cpu_die never frees the vp_assit page. It merely disables it.
hv_cpu_init only allocates a new page if necessary.

Wei.

> 
> -- 
> Vitaly
>