RE: [PATCH v5] video: hyperv: hyperv_fb: Support deferred IO for Hyper-V frame buffer driver

[Dexuan Cui <decui@microsoft.com>]

Thanks,
-- Dexuan

> -----Original Message-----
> From: linux-hyperv-owner@vger.kernel.org
> <linux-hyperv-owner@vger.kernel.org> On Behalf Of Dexuan Cui
> Sent: Thursday, September 12, 2019 11:38 PM
> To: Wei Hu <weh@microsoft.com>; Michael Kelley <mikelley@microsoft.com>;
> rdunlap@infradead.org; shc_work@mail.ru; gregkh@linuxfoundation.org;
> lee.jones@linaro.org; alexandre.belloni@bootlin.com;
> baijiaju1990@gmail.com; fthain@telegraphics.com.au; info@metux.net;
> linux-hyperv@vger.kernel.org; dri-devel@lists.freedesktop.org;
> linux-fbdev@vger.kernel.org; linux-kernel@vger.kernel.org; sashal@kernel.org;
> Stephen Hemminger <sthemmin@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; KY Srinivasan <kys@microsoft.com>
> Subject: RE: [PATCH v5] video: hyperv: hyperv_fb: Support deferred IO for
> Hyper-V frame buffer driver
> 
> > From: Wei Hu <weh@microsoft.com>
> > Sent: Thursday, September 12, 2019 11:03 PM
> >
> > 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 there
> > 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.
> >
> > Currently Hyper-V only takes a physical address from guest as the starting
> > 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 deferred
> > IO outweighs the shadow copy impact in the guest.
> >
> > 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 sending
> >     to Hyper-V.
> >     - Corrected the dirty rectangle size in the text mode.
> >     - Added more comments.
> >     - Other minor code cleanups.
> >
> >     v3: Incorporated more review comments
> >     - Removed a few unnecessary variable tests
> >
> >     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.
> >
> >     v5: Completely remove the unnecessary docopy_lock after discussing
> >     with Dexuan Cui.
> 
> Thanks! Looks good to me.
> 
> Reviewed-by: Dexuan Cui <decui@microsoft.com>


Hi Wei,
It turns out we need to make a further fix. :-)

The patch forgets to take par->update into consideration.

When the VM Connection window is closed (or minimized?),
the host sends a message to the guest, and the guest sets
par->update to false in synthvid_recv_sub().

If par->update is false, the guest doesn't need to call
synthvid_update().

Thanks,
-- Dexuan