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

[Dexuan Cui]

> From: Wei Hu <weh at microsoft.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 at microsoft.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 at microsoft.com>