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

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Date:   Mon, 16 Sep 2019 21:46:35 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Dexuan Cui <decui@...rosoft.com>, Wei Hu <weh@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "shc_work@...l.ru" <shc_work@...l.ru>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "lee.jones@...aro.org" <lee.jones@...aro.org>,
        "alexandre.belloni@...tlin.com" <alexandre.belloni@...tlin.com>,
        "baijiaju1990@...il.com" <baijiaju1990@...il.com>,
        "fthain@...egraphics.com.au" <fthain@...egraphics.com.au>,
        "info@...ux.net" <info@...ux.net>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "sashal@...nel.org" <sashal@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>
Subject: RE: [PATCH v5] video: hyperv: hyperv_fb: Support deferred IO for
 Hyper-V frame buffer driver



Thanks,
-- Dexuan

> -----Original Message-----
> From: linux-hyperv-owner@...r.kernel.org
> <linux-hyperv-owner@...r.kernel.org> On Behalf Of Dexuan Cui
> Sent: Thursday, September 12, 2019 11:38 PM
> To: Wei Hu <weh@...rosoft.com>; Michael Kelley <mikelley@...rosoft.com>;
> rdunlap@...radead.org; shc_work@...l.ru; gregkh@...uxfoundation.org;
> lee.jones@...aro.org; alexandre.belloni@...tlin.com;
> baijiaju1990@...il.com; fthain@...egraphics.com.au; info@...ux.net;
> linux-hyperv@...r.kernel.org; dri-devel@...ts.freedesktop.org;
> linux-fbdev@...r.kernel.org; linux-kernel@...r.kernel.org; sashal@...nel.org;
> Stephen Hemminger <sthemmin@...rosoft.com>; Haiyang Zhang
> <haiyangz@...rosoft.com>; KY Srinivasan <kys@...rosoft.com>
> Subject: RE: [PATCH v5] video: hyperv: hyperv_fb: Support deferred IO for
> Hyper-V frame buffer driver
> 
> > From: Wei Hu <weh@...rosoft.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@...rosoft.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@...rosoft.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