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

From: Wei Hu <redacted> Sent: Tuesday, September 17, 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 <redacted>
---
    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.

    v6: Do not request host refresh when the VM guest screen is
    closed or minimized.

 drivers/video/fbdev/Kconfig     |   1 +
 drivers/video/fbdev/hyperv_fb.c | 210 ++++++++++++++++++++++++++++----
 2 files changed, 190 insertions(+), 21 deletions(-)

Reviewed-by: Michael Kelley <redacted>