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Date:   Mon, 25 Jul 2022 18:55:20 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
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Subject: RE: [PATCH v2 12/13] drivers/hv/vmbus, video/hyperv_fb: Untangle and
 refactor Hyper-V panic notifiers

From: Guilherme G. Piccoli <gpiccoli@...lia.com> Sent: Tuesday, July 19, 2022 12:53 PM
> 
> Currently Hyper-V guests are among the most relevant users of the panic
> infrastructure, like panic notifiers, kmsg dumpers, etc. The reasons rely
> both in cleaning-up procedures (closing hypervisor <-> guest connection,
> disabling some paravirtualized timer) as well as to data collection
> (sending panic information to the hypervisor) and framebuffer management.
> 
> The thing is: some notifiers are related to others, ordering matters, some
> functionalities are duplicated and there are lots of conditionals behind
> sending panic information to the hypervisor. As part of an effort to
> clean-up the panic notifiers mechanism and better document things, we
> hereby address some of the issues/complexities of Hyper-V panic handling
> through the following changes:
> 
> (a) We have die and panic notifiers on vmbus_drv.c and both have goals of
> sending panic information to the hypervisor, though the panic notifier is
> also responsible for a cleaning-up procedure.
> 
> This commit clears the code by splitting the panic notifier in two, one
> for closing the vmbus connection whereas the other is only for sending
> panic info to hypervisor. With that, it was possible to merge the die and
> panic notifiers in a single/well-documented function, and clear some
> conditional complexities on sending such information to the hypervisor.
> 
> (b) There is a Hyper-V framebuffer panic notifier, which relies in doing
> a vmbus operation that demands a valid connection. So, we must order this
> notifier with the panic notifier from vmbus_drv.c, to guarantee that the
> framebuffer code executes before the vmbus connection is unloaded.
> 
> Also, this commit removes a useless header.
> 
> Although there is code rework and re-ordering, we expect that this change
> has no functional regressions but instead optimize the path and increase
> panic reliability on Hyper-V. This was tested on Hyper-V with success.
> 
> Cc: Andrea Parri (Microsoft) <parri.andrea@...il.com>
> Cc: Dexuan Cui <decui@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
> Cc: Michael Kelley <mikelley@...rosoft.com>
> Cc: Petr Mladek <pmladek@...e.com>
> Cc: Stephen Hemminger <sthemmin@...rosoft.com>
> Cc: Tianyu Lan <Tianyu.Lan@...rosoft.com>
> Cc: Wei Liu <wei.liu@...nel.org>
> Tested-by: Fabio A M Martins <fabiomirmar@...il.com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
> 
> ---
> 
> 
> V2:
> - Unfortunately we cannot rely in the crash shutdown (custom) handler
> to perform the vmbus unload - arm64 architecture doesn't have this
> "feature" [0]. So, in V2 we kept the notifier behavior and always
> unload the vmbus connection, no matter what - thanks Michael for
> pointing that;
> 
> - Removed the Fixes tags as per Michael suggestion;
> 
> - As per Petr suggestion, we abandoned the idea of distinguish among
> notifiers using an id - so, in V2 we rely in the old and good address
> comparison for that. Thanks Petr for the enriching discussion!
> 
> [0]
> https://lore.kernel.org/lkml/427a8277-49f0-4317-d6c3-4a15d7070e55@igalia.com/
> 
> 
>  drivers/hv/vmbus_drv.c          | 109 +++++++++++++++++++-------------
>  drivers/video/fbdev/hyperv_fb.c |   8 +++
>  2 files changed, 74 insertions(+), 43 deletions(-)
> 

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

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