[alsa-devel] Getting rid of inside_vm in intel8x0

[Takashi Iwai]

On Fri, 01 Apr 2016 00:26:18 +0200,
Luis R. Rodriguez wrote:
> 
> On Wed, Mar 30, 2016 at 08:07:04AM +0200, Takashi Iwai wrote:
> > On Tue, 29 Mar 2016 23:37:32 +0200,
> > Andy Lutomirski wrote:
> > > 
> > > Would it be possible to revert:
> > > 
> > > commit 228cf79376f13b98f2e1ac10586311312757675c
> > > Author: Konstantin Ozerkov <kozerkov at parallels.com>
> > > Date:   Wed Oct 26 19:11:01 2011 +0400
> > > 
> > >     ALSA: intel8x0: Improve performance in virtual environment
> > > 
> > > Presumably one or more of the following is true:
> > > 
> > > a) The inside_vm == true case is just an optimization and should apply
> > > unconditionally.
> > > 
> > > b) The inside_vm == true case is incorrect and should be fixed or disabled.
> > > 
> > > 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?
> 
> BTW can we simulate this on bare metal by throttling an IO bus, or
> perhaps mucking with the scheduler ?
> 
> I ask as I wonder if similar type of optimization may be useful
> to first simulate with other types of buses for other IO devices
> we might use in virtualization environments. If so, I'd be curious
> to know if similar type of optimizations might be possible for
> other sounds cards, or other IO devices.

There aren't so many sound devices requiring such a workaround.

> > More important condition is rather that the register updates of CIV
> > and PICB are atomic.
> 
> To help with this can you perhaps elaborate a bit more on what the code
> does? As I read it snd_intel8x0_pcm_pointer() gets a pointer to some
> sort of audio frame we are in and uses two values to see if we are
> going to be evaluating the right frame, we use an optimization of
> some sort to skip one check for virtual environments. We seem to need
> this given that on a virtual environment it is assumed that the sound
> card is emulated, and as such an IO read there is rather expensive.
> 
> Can you confirm and/or elaborate a bit more what this does ?
> 
> To try to help understand what is going on can you describe what CIV
> and PICB are exactly ?

CIV and PICB registers are a pair and we calculate the linear position
in a ring buffer from both two.  However, they are divorced sometimes
under stress, and the position calculated from such values may go
backward wrongly.  For avoiding it, there is the second read of the
PICB register and compare with the previous value, and loop until it
matches.  This check is skipped on VM.

> > This is satisfied mostly only on VM, and can't
> > be measured easily unlike the IO read speed.
> 
> Interesting, note the original patch claimed it was for KVM and
> Parallels hypervisor only, but since the code uses:
> 
> +#if defined(__i386__) || defined(__x86_64__)
> +               inside_vm = inside_vm || boot_cpu_has(X86_FEATURE_HYPERVISOR);
> +#endif
> 
> This makes it apply also to Xen as well, this makes this hack more
> broad, but does is it only applicable when an emulated device is
> used ? What about if a hypervisor is used and PCI passthrough is
> used ?

A good question.  Xen was added there at the time from positive
results by quick tests, but it might show an issue if it's running on
a very old chip with PCI passthrough.  But I'm not sure whether PCI
passthrough would work on such old chipsets at all.

> > > 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 :)
> > 
> > This cake looks sweet, but a worm is hidden behind the cream.
> > The loop in the code itself is already a kludge for the buggy hardware
> > where the inconsistent read happens not so often (only at the boundary
> > and in a racy way).  It would be nice if we can have a more reliably
> > way to know the hardware buggyness, but it's difficult,
> > unsurprisingly.
> 
> The concern here is setting precedents for VM cases sprinkled in the kernel.
> The assumption here is such special cases are really paper'ing over another
> type of issue, so its best to ultimately try to root cause the issue in
> a more generalized fashion.
 
Well, it's rather bare metal that shows the buggy behavior, thus we
need to paper over it.   In that sense, it's other way round; we don't
tune for VM.  The VM check we're discussing is rather for skipping the
strange workaround.

You may ask whether we can reduce the whole workaround instead.  It's
practically impossible.  We don't know which models doing so and which
not.  And, the hardware in question are (literally) thousands of
variants of damn old PC mobos.  Any fundamental change needs to be
verified on all these machines...


Takashi