From: Michael Kelley <redacted>  Sent: Wednesday, September 18, 2019 2:48 PM

quoted

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 <redacted>

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