Speed up vt(4) by keeping a record of the most recently drawn character and
the foreground and background colours. In bitblt_text functions, compare
values to this cache and don't re-draw the charac

Speed up vt(4) by keeping a record of the most recently drawn character and
the foreground and background colours. In bitblt_text functions, compare
values to this cache and don't re-draw the characters if they haven't changed.
When invalidating the display, clear this cache in order to force characters
to be redrawn; also force full redraws between suspend/resume pairs since odd
artifacts can otherwise result.

When scrolling the display (which is where most time is spent within the vt
driver) this yields a significant performance improvement if most lines are
less than the width of the terminal, since this avoids re-drawing blanks on
top of blanks.

(Note that "re-drawing" here includes writing to the VGA text mode buffer; on
virtualized systems this can be extremely slow since it triggers a glyph
being rendered onto a 640x480 screen).

On a c5.4xlarge EC2 instance (with emulated text mode VGA) this cuts the time
spent in vt(4) during the kernel boot from 1200 ms to 700ms; on my laptop
(with a 3200x1800 display) the corresponding time is reduced from 970 ms down
to 155 ms.

Reviewed by: imp, cem
Approved by: re (gjb)
Relnotes: Significant speedup in vt(4) and the system boot generally.
Differential Revision: https://reviews.freebsd.org/D16723

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Replace a pair of 8-bit writes to VGA memory with a single 16-bit write.

The VGA "text mode" buffer has a pair of bytes for each character: One
byte for the character symbol, and an "attribute" byte

Replace a pair of 8-bit writes to VGA memory with a single 16-bit write.

The VGA "text mode" buffer has a pair of bytes for each character: One
byte for the character symbol, and an "attribute" byte encoding the
foreground and background colours. When updating the screen, we were
writing these two bytes separately.

On some virtualized systems, every write results in a glyph being redrawn
into a (graphical) virtual screen; writing these two bytes separately
results in twice as much work being done to draw characters, whereas if
we perform a single 16-bit write instead, the character only needs to be
redrawn once.

On an EC2 c5.4xlarge instance, this change cuts 1.30s from the kernel boot,
speeding it up from 8.90s to 7.60s.

MFC after: 1 week

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