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From:   Michael Kelley <mikelley@microsoft.com>
To:     "Andrea Parri (Microsoft)" <parri.andrea@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        Dexuan Cui <decui@microsoft.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        vkuznets <vkuznets@redhat.com>
Subject: RE: [PATCH 10/11] Drivers: hv: vmbus: Introduce the
 CHANNELMSG_MODIFYCHANNEL message type
Thread-Topic: [PATCH 10/11] Drivers: hv: vmbus: Introduce the
 CHANNELMSG_MODIFYCHANNEL message type
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Date:   Fri, 10 Apr 2020 19:34:53 +0000
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 <20200406001514.19876-11-parri.andrea@gmail.com>
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From: Andrea Parri (Microsoft) <parri.andrea@gmail.com> Sent: Sunday, April=
 5, 2020 5:15 PM
>=20
> VMBus version 4.1 and later support the CHANNELMSG_MODIFYCHANNEL(22)
> message type which can be used to request Hyper-V to change the vCPU
> that a channel will interrupt.
>=20
> Introduce the CHANNELMSG_MODIFYCHANNEL message type, and define the
> vmbus_send_modifychannel() function to send CHANNELMSG_MODIFYCHANNEL
> requests to the host via a hypercall.  The function is then used to
> define a sysfs "store" operation, which allows to change the (v)CPU
> the channel will interrupt by using the sysfs interface.  The feature
> can be used for load balancing or other purposes.
>=20
> One interesting catch here is that Hyper-V can *not* currently ACK
> CHANNELMSG_MODIFYCHANNEL messages with the promise that (after the ACK
> is sent) the channel won't send any more interrupts to the "old" CPU.
>=20
> The peculiarity of the CHANNELMSG_MODIFYCHANNEL messages is problematic
> if the user want to take a CPU offline, since we don't want to take a
> CPU offline (and, potentially, "lose" channel interrupts on such CPU)
> if the host is still processing a CHANNELMSG_MODIFYCHANNEL message
> associated to that CPU.
>=20
> It is worth mentioning, however, that we have been unable to observe
> the above mentioned "race": in all our tests, CHANNELMSG_MODIFYCHANNEL
> requests appeared *as if* they were processed synchronously by the host.
>=20
> Suggested-by: Michael Kelley <mikelley@microsoft.com>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> ---
>  drivers/hv/channel.c      |  28 ++++++++++
>  drivers/hv/channel_mgmt.c |   2 +-
>  drivers/hv/hv_trace.h     |  19 +++++++
>  drivers/hv/vmbus_drv.c    | 108 +++++++++++++++++++++++++++++++++++++-
>  include/linux/hyperv.h    |  10 +++-
>  5 files changed, 163 insertions(+), 4 deletions(-)
>=20

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