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

[Michael Kelley]

From: Wei Hu <weh@xxxxxxxxxxxxx> 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 <weh@xxxxxxxxxxxxx>
> ---
>     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 <mikelley@xxxxxxxxxxxxx>