RE: [PATCH v2] x86/hyperv: Fix kexec panic/hang issues

[Michael Kelley]

From: Wei Liu <wei.liu@xxxxxxxxxx> Sent: Tuesday, January 5, 2021 5:04 AM
> 
> 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 the
> > 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 CPUs
> > are stopped in hv_machine_shutdown() -> native_machine_shutdown(), so
> > between hv_kexec_handler() and native_machine_shutdown(), the other CPUs
> > can still try to access the hypercall page and cause panic. The workaround
> > "hv_hypercall_pg = NULL;" in hyperv_cleanup() is unreliabe. Move
> > hyperv_cleanup() to a better place.
> >
> > Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> 
> The code looks a bit intrusive. On the other hand, this does sound like
> something needs backporting for older stable kernels.
> 
> On a more practical note, I need to decide whether to take it via
> hyperv-fixes or hyperv-next. What do you think?
> 

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 problems
that make it unreliable, so the downside risk of taking these fixes is minimal
while the upside benefit is considerable.

Michael