On Mon, 2013-06-24 at 11:43 -0600, Bjorn Helgaas wrote: > On Wed, Jun 19, 2013 at 6:43 AM, Don Dutile <ddutile@xxxxxxxxxx> wrote: > > On 06/18/2013 10:52 PM, Bjorn Helgaas wrote: > >> > >> On Tue, Jun 18, 2013 at 5:03 PM, Don Dutile<ddutile@xxxxxxxxxx> wrote: > >>> > >>> On 06/18/2013 06:22 PM, Alex Williamson wrote: > >>>> > >>>> > >>>> On Tue, 2013-06-18 at 15:31 -0600, Bjorn Helgaas wrote: > >>>>> > >>>>> > >>>>> On Tue, Jun 18, 2013 at 12:20 PM, Alex Williamson > >>>>> <alex.williamson@xxxxxxxxxx> wrote: > >>>>>> > >>>>>> > >>>>>> On Tue, 2013-06-18 at 11:28 -0600, Bjorn Helgaas wrote: > >>>>>>> > >>>>>>> > >>>>>>> On Thu, May 30, 2013 at 12:40:19PM -0600, Alex Williamson wrote: > >>> > >>> ... > >>>>>>> > >>>>>>> Who do you expect to decide whether to use this option? I think it > >>>>>>> requires intimate knowledge of how the device works. > >>>>>>> > >>>>>>> I think the benefit of using the option is that it makes assignment > >>>>>>> of > >>>>>>> devices to guests more flexible, which will make it attractive to > >>>>>>> users. > >>>>>>> But most users have no way of knowing whether it's actually *safe* to > >>>>>>> use this. So I worry that you're adding an easy way to pretend > >>>>>>> isolation > >>>>>>> exists when there's no good way of being confident that it actually > >>>>>>> does. > >> > >> > >>> ... > >> > >> > >>>>> I wonder if we should taint the kernel if this option is used (but not > >>>>> for specific devices added to pci_dev_acs_enabled[]). It would also > >>>>> be nice if pci_dev_specific_acs_enabled() gave some indication in > >>>>> dmesg for the specific devices you're hoping to add to > >>>>> pci_dev_acs_enabled[]. It's not an enumeration-time quirk right now, > >>>>> so I'm not sure how we'd limit it to one message per device. > >>>> > >>>> > >>>> Right, setup vs use and getting single prints is a lot of extra code. > >>>> Tainting is troublesome for support, Don had some objections when I > >>>> suggested the same to him. > >>>> > >>> For RH GSS (Global Support Services), a 'taint' in the kernel printk > >>> means > >>> RH doesn't support that system. The 'non-support' due to 'taint' being > >>> printed > >>> out in this case may be incorrect -- RH may support that use, at least > >>> until > >>> a more sufficient patched kernel is provided. > >>> Thus my dissension that 'taint' be output. WARN is ok. 'driver beware', > >>> 'unleashed dog afoot'.... sure... > >> > >> > >> So ... that's really a RH-specific support issue, and easily worked > >> around by RH adding a patch that turns off tainting. > >> > > sure. what's another patch to the thousands... :-/ > > > >> It still sounds like a good idea to me for upstream, where use of this > >> option can very possibly lead to corruption or information leakage > >> between devices the user claimed were isolated, but in fact were not. > > > > Did I miss something? This patch provides a user-level/chosen override; > > like all other overrides, (pci=realloc, etc.), it can lead to a failing > > system. > > IMO, this patch is no different. If you want to tag this patch with taint, > > then let's audit all the (PCI) overrides and taint them appropriately. > > Taint should be reserved to changes to the kernel that were done outside > > the development of the kernel, or with the explicit intent to circumvent > > the normal operation of the kernel. This patch provides a way to enable > > ACS checking to succeed when the devices have not provided sufficiently > > complete > > ACS information. i.e., it's a growth path for PCIe-ACS and its need for > > proper support. > > We're telling the kernel to assume something (the hardware provides > protection) that may not be true. If that assumption turns out to be > false, the result is that a VM can be crashed or comprised by another > VM. > > One difference I see is that this override can lead to a crash that > looks like random memory corruption and has no apparent connection to > the actual cause. Most other overrides won't cause run-time crashes > (I think they're more likely to cause boot or device configuration > failures), and the dmesg log will probably have good clues as to the > reason. > > But the possibility of compromise is probably even more serious, > because there would be no crash at all, and we'd have no indication > that VM A read or corrupted data in VM B. I'm very concerned about > that, enough so that it's not clear to me that an override belongs in > the upstream kernel at all. > > Yes, that would mean some hardware is not suitable for device > assignment. That just sounds like "if hardware manufacturers do their > homework and support ACS properly, their hardware is more useful for > virtualization than other hardware." I don't see the problem with > that. That's easy to say for someone that doesn't get caught trying to explain this to users over and over. In many cases devices don't do peer-to-peer and missing ACS is an oversight. I imagine that quite a few vendors also see the ACS capability as a means to allow control of ACS and therefore see it as a much larger investment that just providing an empty ACS structure in config space to indicate the lack of peer-to-peer. Even if we taint the kernel when this is enabled and add extremely verbose warnings in kernel-parameters.txt, I think there's value to providing an on-the-spot workaround to users. In many cases we're not going to get a response from vendors. Removing the downstream/multifunction catch-alls might be another way to raise the bar for this kind of override. Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
