Hello.
I'm strongly disagree to revert this path or disable optimization. This patch fixes real sound issue when Linux runs inside VM.
On 30/03/16 00:37, Andy Lutomirski wrote:
Would it be possible to revert:
commit 228cf79376f13b98f2e1ac10586311312757675c Author: Konstantin Ozerkov kozerkov@parallels.com Date: Wed Oct 26 19:11:01 2011 +0400
ALSA: intel8x0: Improve performance in virtual environmentPresumably one or more of the following is true:
a) The inside_vm == true case is just an optimization and should apply unconditionally.
Wrong. Code without optimization gracefully handle some odd hardware. But code with optimization also fixes real problem in virtual environment.
b) The inside_vm == true case is incorrect and should be fixed or disabled.
Could you explain what is wrong ?
c) The inside_vm == true case is a special case that makes sense then IO is very very slow but doesn't make sense when IO is fast. If so, why not literally measure the time that the IO takes and switch over to the "inside VM" path when IO is slow?
Measuring IO performance inside VM is not easy as may seen. Most of clock sources are emulated and adjusted to hide VM-exit (emulation) hole, i.e. time flow inside VM have delays and speedups in comparison to wall clock time. To detect "inside VM" condition by this way loop should run at least 1 seconds or more (10 or more preferred) without guarantee of proper detection. I think this is not acceptable at module initialization time.
There are a pile of nonsensical "are we in a VM" checks of various sorts scattered throughout the kernel, they're all a mess to maintain (there are lots of kinds of VMs in the world, and Linux may not even know it's a guest), and, in most cases, it appears that the correct solution is to delete the checks. I just removed a nasty one in the x86_32 entry asm, and this one is written in C so it should be a piece of cake :)
From drivers view VM is sort of hardware with special requirements and capabilities. Sometimes drivers should be aware about them to work properly.
The simplest way to remove "are we in a VM" checks mess is write kernel-wide service that detects this condition and switch to use it in other places.
I agree that check inside snd_intel8x0_inside_vm() looks very strict. Originally this code was designed to enable optimization only for well tested environments (KVM-based or Parallels VMs).
--Andy