v3: detection code is x86 and KVM specific, hide it under ifdef
v2: add detection for virtual environments (KVM and Parallels)
This patch is intended to improve performance in virtualized environments
like Parallels Desktop or KVM/VirtualBox/QEMU (virtual ICH/AC97 audio).
I/O access is very time-expensive operation in virtual world: VCPU
can be rescheduled and in the worst case we get more than 10ms delay on
each I/O access.
In the virtual environment loop exit rule
(old_civ == current_civ && old_picb == current_picb) is never satisfied,
because old_picb is never the same as current_picb due to delay inspired
by reading current_civ. As a result loop ended by timeout and we get 10x
more I/O operations.
Experimental data from Prallels Desktop 7, RHEL6 guest (I/O ops per
second):
Original code:
In Port Counter Callback
f014 41550 fffff00000179d00 ac97_bm_read_civ+0x000
f018 41387 fffff0000017a580 ac97_bm_read_picb+0x000
With patch:
In Port Counter Callback
f014 4090 fffff00000179d00 ac97_bm_read_civ+0x000
f018 1964 fffff0000017a580 ac97_bm_read_picb+0x000
Signed-off-by: Konstantin Ozerkov <kozerkov@parallels.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
228cf79376