Re: [Xen-devel] Getting rid of inside_vm in intel8x0

[Takashi Iwai <tiwai@xxxxxxx>]

On Mon, 04 Apr 2016 11:05:43 +0200,
George Dunlap wrote:
> 
> On 02/04/16 13:57, Andy Lutomirski wrote:
> > On Fri, Apr 1, 2016 at 10:33 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> >> On Sat, 02 Apr 2016 00:28:31 +0200,
> >> Luis R. Rodriguez wrote:
> >>> If the former, could a we somehow detect an emulated device other than 
> >>> through
> >>> this type of check ? Or could we *add* a capability of some sort to 
> >>> detect it
> >>> on the driver ? This would not address the removal, but it could mean 
> >>> finding a
> >>> way to address emulation issues.
> >>>
> >>> If its an IO issue -- exactly what is the causing the delays in IO ?
> >>
> >> Luis, there is no problem about emulation itself.  It's rather an
> >> optimization to lighten the host side load, as I/O access on a VM is
> >> much heavier.
> >>
> >>>>>> 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.
> >>>
> >>> If it did have an issue then that would have to be special cased, that
> >>> is the module parameter would not need to be enabled for such type of
> >>> systems, and heuristics would be needed. As you note, fortunately this
> >>> may not be common though...
> >>
> >> Actually this *is* module parametered.  If set to a boolean value, it
> >> can be applied / skipped forcibly.  So, if there has been a problem on
> >> Xen, this should have been reported.  That's why I wrote it's no
> >> common case.  This comes from the real experience.
> >>
> >>> but if this type of work around may be
> >>> taken as a precedent to enable other types of hacks in other drivers
> >>> I'm very fearful of more hacks later needing these considerations as
> >>> well.
> >>>
> >>>>>>> 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.
> >>>
> >>> What is it exactly about a VM that enables this work around to be skipped?
> >>> I don't quite get it yet.
> >>
> >> VM -- at least the full one with the sound hardware emulation --
> >> doesn't have the hardware bug.  So, the check isn't needed.
> > 
> > Here's the issue, though: asking "am I in a VM" is not a good way to
> > learn properties of hardware.  Just off the top of my head, here are
> > some types of VM and what they might imply about hardware:
> > 
> > Intel Kernel Guard: your sound card is passed through from real hardware.
> > 
> > Xen: could go either way.  In dom0, it's likely passed through.  In
> > domU, it could be passed through or emulated, and I believe this is
> > the case for all of the Xen variants.
> > 
> > KVM: Probably emulated, but could be passed through.
> 
> I'm not sure exactly why I was CC'd into this thread, but this is an
> important point -- even if you're running in a VM, you may actually have
> direct un-emulated IO access to a real (buggy) piece of hardware; in
> which case it sounds like you still need the work-around.  So
> boot_cpu_has(X86_FEATURE_HYPERVISOR) is probably not the right check.

The VM check is kept there only to show a kernel message; in case
where a similar issue is seen on another VM, user may notice more
easily by that.  The VM check itself doesn't change any kernel
behavior any longer.


Takashi

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