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Date:   Fri, 17 Apr 2020 14:08:03 +0100
From:   Wei Liu <wei.liu@kernel.org>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Wei Liu <wei.liu@kernel.org>, 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, kvm@vger.kernel.org
Subject: Re: [PATCH] x86/hyperv: Suspend/resume the VP assist page for
 hibernation
Message-ID: <20200417130803.ci2orezcrb64z64m@debian>
References: <1587104999-28927-1-git-send-email-decui@microsoft.com>
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 <20200417105558.2jkqq2lih6vvoip2@debian>
 <87wo6etj39.fsf@vitty.brq.redhat.com>
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On Fri, Apr 17, 2020 at 02:03:38PM +0200, Vitaly Kuznetsov wrote:
> Wei Liu <wei.liu@kernel.org> writes:
> 
> > 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?
> >
> 
> Right, unlike hyperv_pcpu_input_arg is is not freed.
> 
> > hv_cpu_die never frees the vp_assit page. It merely disables it.
> > hv_cpu_init only allocates a new page if necessary.
> 
> I'm not really sure that Hyper-V will like us when we disable VP Assist
> page and have an active L2 guest using Enlightened VMCS, who knows what
> it caches and when. I'll try to at least test if/how it works.
> 

I'm curious to know that as well. :-)

> This all is not really related to Dexuan's patch)

Right.

Wei.

> 
> -- 
> Vitaly
>