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From:   Michael Kelley <mikelley@microsoft.com>
To:     Dexuan Cui <decui@microsoft.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>,
        "sashal@kernel.org" <sashal@kernel.org>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v4 02/12] x86/hyper-v: Implement
 hv_is_hibernation_supported()
Thread-Topic: [PATCH v4 02/12] x86/hyper-v: Implement
 hv_is_hibernation_supported()
Thread-Index: AQHVYe3JrHu+0vx33Ua7p5urdffONKcbu4lw
Date:   Wed, 4 Sep 2019 16:43:15 +0000
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From: Dexuan Cui <decui@microsoft.com> Sent: Monday, September 2, 2019 5:23=
 PM
>=20
> The API will be used by the hv_balloon and hv_vmbus drivers.
>=20
> Balloon up/down and hot-add of memory must not be active if the user
> wants the Linux VM to support hibernation, because they are incompatible
> with hibernation according to Hyper-V team, e.g. upon suspend the
> balloon VSP doesn't save any info about the ballooned-out pages (if any);
> so, after Linux resumes, Linux balloon VSC expects that the VSP will
> return the pages if Linux is under memory pressure, but the VSP will
> never do that, since the VSP thinks it never stole the pages from the VM.
>=20
> So, if the user wants Linux VM to support hibernation, Linux must forbid
> balloon up/down and hot-add, and the only functionality of the balloon VS=
C
> driver is reporting the VM's memory pressure to the host.
>=20
> Ideally, when Linux detects that the user wants it to support hibernation=
,
> the balloon VSC should tell the VSP that it does not support ballooning
> and hot-add. However, the current version of the VSP requires the VSC
> should support these capabilities, otherwise the capability negotiation
> fails and the VSC can not load at all, so with the later changes to the
> VSC driver, Linux VM still reports to the VSP that the VSC supports these
> capabilities, but the VSC ignores the VSP's requests of balloon up/down
> and hot add, and reports an error to the VSP, when applicable. BTW, in
> the future the balloon VSP driver will allow the VSC to not support the
> capabilities of balloon up/down and hot add.
>=20
> The ACPI S4 state is not a must for hibernation to work, because Linux is
> able to hibernate as long as the system can shut down. However in practic=
e
> we decide to artificially use the presence of the virtual ACPI S4 state a=
s
> an indicator of the user's intent of using hibernation, because Linux VM
> must find a way to know if the user wants to use the hibernation feature
> or not.
>=20
> By default, Hyper-V does not enable the virtual ACPI S4 state; on recent
> Hyper-V hosts (e.g. RS5, 19H1), the administrator is able to enable the
> state for a VM by WMI commands.
>=20
> Once all the vmbus and VSC patches for the hibernation feature are
> accepted, an extra patch will be submitted to forbid hibernation if the
> virtual ACPI S4 state is absent, i.e. hv_is_hibernation_supported() is
> false.
>=20
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>  arch/x86/hyperv/hv_init.c      | 7 +++++++
>  include/asm-generic/mshyperv.h | 2 ++
>  2 files changed, 9 insertions(+)

Reviewed-by: Michael Kelley <mikelley@microsoft.com>