On Fri, 2014-01-31 at 11:49 -0700, Bjorn Helgaas wrote: > On Mon, Jan 20, 2014 at 03:01:40PM -0700, Alex Williamson wrote: > > As described in 2/2 many Intel root ports lack PCIe ACS capabilities > > which results in excessively large IOMMU groups. Many of these root > > ports do provide isolation capabilities, we just need to use device > > specific mechanisms to enable and verify. Long term, I hope we can > > round out this list (particularly to include X79 root ports) and > > more importantly, encourage proper PCIe ACS support in future > > products. I'm really hoping we can get this in during the 3.14 cycle. > > v3.13 was released Jan 19, so this came during the v3.14 merge window. I > like to have things in -next for a while before asking Linus to pull them, > and I try to keep it to regression fixes after the merge window, i.e., > things that used to work, but don't work any more. But that's all standard > procedure that you already know, and maybe you can make a case for > accelerating this. Sorry, I sent it out as early as I was able to. It's my understanding that fixes are always allowed after the merge window and we can generally assume that 3.14 won't be released for at least 6-8 weeks after rc1, which gives us plenty of time to let this cook in -next and still make 3.14. Obviously it's your prerogative as maintainer where you feel comfortable. So what does this fix and how close is it to a regression? The fix is a quirk for hardware that lacks proper ACS support, but still provides device isolation when properly configured. What that means to a user is that if they attempt to use vfio to expose a device to a userspace driver such as QEMU, the device may be artificially tied to other devices behind the root port and devices behind root ports that are part of the same multifunction PCH package. In effect, there are devices that are isolated or isolate-able that users should be able to use independently of other devices, but they can't. I fully acknowledge that this use case is a small, but important to me, subset of users. As to a regression, the only remote software regression is that legacy KVM device assignment takes a much more lax (non-existent) approach to kernel-base isolation and allows such assignments. That's a weak regression, but if you're a downstream trying to switch users over to vfio-based device assignment, it's an important one. In some respects there's also a hardware regression. Intel root ports used to support ACS and for whatever reasons they forgot how critical ACS is in determining isolation sets for exposing devices to VMs. We obviously can't go back and fix the existing hardware, but we can fix this regression for many of those chipsets in software (and beat Intel to include it in next generation hardware). I hope you'll consider it for 3.14, I know a number of users who continue to patch their kernel with the old ACS override patch who would appreciate it sooner than later, but I fully understand wanting some soak time in -next first. 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
