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From:   Michael Kelley <mikelley@microsoft.com>
To:     Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>
CC:     "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>
Subject: RE: [PATCH v2] x86/hyperv: Fix kexec panic/hang issues
Thread-Topic: [PATCH v2] x86/hyperv: Fix kexec panic/hang issues
Thread-Index: AQHW2C+JeK2pk6EyK0KhohVBANde/KoZFjSAgAA6vWA=
Date:   Tue, 5 Jan 2021 16:39:38 +0000
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From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, January 5, 2021 5:04 AM
>=20
> On Mon, Dec 21, 2020 at 10:55:41PM -0800, Dexuan Cui wrote:
> > Currently the kexec kernel can panic or hang due to 2 causes:
> >
> > 1) hv_cpu_die() is not called upon kexec, so the hypervisor corrupts th=
e
> > old VP Assist Pages when the kexec kernel runs. The same issue is fixed
> > for hibernation in commit 421f090c819d ("x86/hyperv: Suspend/resume the
> > VP assist page for hibernation"). Now fix it for kexec.
> >
> > 2) hyperv_cleanup() is called too early. In the kexec path, the other C=
PUs
> > are stopped in hv_machine_shutdown() -> native_machine_shutdown(), so
> > between hv_kexec_handler() and native_machine_shutdown(), the other CPU=
s
> > can still try to access the hypercall page and cause panic. The workaro=
und
> > "hv_hypercall_pg =3D NULL;" in hyperv_cleanup() is unreliabe. Move
> > hyperv_cleanup() to a better place.
> >
> > Signed-off-by: Dexuan Cui <decui@microsoft.com>
>=20
> The code looks a bit intrusive. On the other hand, this does sound like
> something needs backporting for older stable kernels.
>=20
> On a more practical note, I need to decide whether to take it via
> hyperv-fixes or hyperv-next. What do you think?
>=20

I'd like to see this in hyperv-fixes and backported to older stable kernels=
.
In its current form, the kexec path in a Hyper-V guest has multiple problem=
s
that make it unreliable, so the downside risk of taking these fixes is mini=
mal
while the upside benefit is considerable.

Michael