[alsa-devel] Getting rid of inside_vm in intel8x0
Luis R. Rodriguez
mcgrof at suse.com
Mon Apr 4 20:31:03 CEST 2016
On Sat, Apr 02, 2016 at 10:22:44PM +0200, Takashi Iwai wrote:
> On Sat, 02 Apr 2016 20:05:21 +0200,
> Andy Lutomirski wrote:
> >
> > On Apr 2, 2016 12:07 PM, "Takashi Iwai" <tiwai at suse.de> wrote:
> > >
> > > On Sat, 02 Apr 2016 14:57:44 +0200,
> > > Andy Lutomirski wrote:
> > > >
> > > > On Fri, Apr 1, 2016 at 10:33 PM, Takashi Iwai <tiwai at suse.de> 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 think the main reason that Luis and I are both uncomfortable with
> > > > "am I in a VM" checks is that they're rarely the right thing to be
> > > > detecting, the APIs are poorly designed, and most of the use cases in
> > > > the kernel are using them as a proxy for something else and would be
> > > > clearer and more future proof if they tested what they actually need
> > > > to test more directly.
> > >
> > > Please, guys, take a look at the code more closely. This is applied
> > > only to the known emulated PCI devices, and the driver shows the
> > > kernel message:
> > >
> > > static int snd_intel8x0_inside_vm(struct pci_dev *pci)
> > > ....
> > > /* check for known (emulated) devices */
> > > if (pci->subsystem_vendor == PCI_SUBVENDOR_ID_REDHAT_QUMRANET &&
> > > pci->subsystem_device == PCI_SUBDEVICE_ID_QEMU) {
> > > /* KVM emulated sound, PCI SSID: 1af4:1100 */
> > > msg = "enable KVM";
> > > } else if (pci->subsystem_vendor == 0x1ab8) {
> > > /* Parallels VM emulated sound, PCI SSID: 1ab8:xxxx */
> > > msg = "enable Parallels VM";
> > > } else {
> > > msg = "disable (unknown or VT-d) VM";
> > > result = 0;
> > > }
> >
> > Now I'm more confused. Why are you checking the PCI IDs *and* whether
> > a hypervisor is detected? Why not check only the IDs?
> >
> > In any event, at the very least the comment is misleading:
> >
> > /* detect KVM and Parallels virtual environments */
> > result = kvm_para_available();
> > #ifdef X86_FEATURE_HYPERVISOR
> > result = result || boot_cpu_has(X86_FEATURE_HYPERVISOR);
> > #endif
> >
> > You're detecting KVM (sometimes) and the x86 "hypervisor" bit. The
> > latter has no particularly well-defined meaning. You're also missing
> > Xen PV, I believe, and I think that Xen PV + QEMU is a real thing, and
> > you'll fail to detect it, even though it can present a QEMU-emulated
> > card.
Indeed, I think some Xen instances would fail to check with this.
> > In other words, how is this code any different from a simple whitelist
> > of two specific cards that work a little differently from others?
>
> The PCI ID whitelist was introduced later in the commit 7fb4f392bd27,
> and before that, we relied on the VM detection as a switch for
> skipping the workaround. The VM detection code was kept there just to
> be sure, in case the whitelist isn't 100% correct.
>
> Looking at the current status, the whitelist alone seems enough, so
> the VM detection code could be dropped, I suppose.
Great, such a change should perhaps highligh the impact also to Xen
then.
Luis
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