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From:   Michael Kelley <mikelley@microsoft.com>
To:     Wei Hu <weh@microsoft.com>,
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        "sashal@kernel.org" <sashal@kernel.org>,
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        Haiyang Zhang <haiyangz@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>
Subject: RE: [PATHC v6] video: hyperv: hyperv_fb: Support deferred IO for
 Hyper-V frame buffer driver
Thread-Topic: [PATHC v6] video: hyperv: hyperv_fb: Support deferred IO for
 Hyper-V frame buffer driver
Thread-Index: AQHVbebF21J2EyXx3k2x6FCXKaeI5qcx+W9w
Date:   Wed, 18 Sep 2019 21:48:04 +0000
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References: <20190918060227.6834-1-weh@microsoft.com>
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From: Wei Hu <weh@microsoft.com> Sent: Tuesday, September 17, 2019 11:03 PM

>=20
> Without deferred IO support, hyperv_fb driver informs the host to refresh
> the entire guest frame buffer at fixed rate, e.g. at 20Hz, no matter ther=
e
> is screen update or not. This patch supports deferred IO for screens in
> graphics mode and also enables the frame buffer on-demand refresh. The
> highest refresh rate is still set at 20Hz.
>=20
> Currently Hyper-V only takes a physical address from guest as the startin=
g
> address of frame buffer. This implies the guest must allocate contiguous
> physical memory for frame buffer. In addition, Hyper-V Gen 2 VMs only
> accept address from MMIO region as frame buffer address. Due to these
> limitations on Hyper-V host, we keep a shadow copy of frame buffer
> in the guest. This means one more copy of the dirty rectangle inside
> guest when doing the on-demand refresh. This can be optimized in the
> future with help from host. For now the host performance gain from deferr=
ed
> IO outweighs the shadow copy impact in the guest.
>=20
> Signed-off-by: Wei Hu <weh@microsoft.com>
> ---
>     v2: Incorporated review comments from Michael Kelley
>     - Increased dirty rectangle by one row in deferred IO case when sendi=
ng
>     to Hyper-V.
>     - Corrected the dirty rectangle size in the text mode.
>     - Added more comments.
>     - Other minor code cleanups.
>=20
>     v3: Incorporated more review comments
>     - Removed a few unnecessary variable tests
>=20
>     v4: Incorporated test and review feedback from Dexuan Cui
>     - Not disable interrupt while acquiring docopy_lock in
>       hvfb_update_work(). This avoids significant bootup delay in
>       large vCPU count VMs.
>=20
>     v5: Completely remove the unnecessary docopy_lock after discussing
>     with Dexuan Cui.
>=20
>     v6: Do not request host refresh when the VM guest screen is
>     closed or minimized.
>=20
>  drivers/video/fbdev/Kconfig     |   1 +
>  drivers/video/fbdev/hyperv_fb.c | 210 ++++++++++++++++++++++++++++----
>  2 files changed, 190 insertions(+), 21 deletions(-)
>=20

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