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@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@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