[PATHC v6] video: hyperv: hyperv_fb: Support deferred IO for Hyper-V frame buffer driver

[Michael Kelley]

From: Michael Kelley <mikelley at microsoft.com>  Sent: Wednesday, September 18, 2019 2:48 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 at microsoft.com>

Sasha -- this patch and one other from Wei Hu for the Hyper-V frame buffer
driver should be ready.  Both patches affect only the Hyper-V frame buffer
driver so can go through the Hyper-V tree.  Can you pick these up?  Thx.

Michael