On Tue, Jul 27, 2021 at 10:30:40PM +0800, Dongdong Liu wrote: > > > On 2021/7/27 19:05, Leon Romanovsky wrote: > > On Mon, Jul 26, 2021 at 09:48:57AM -0600, Logan Gunthorpe wrote: > > > > > > > > > On 2021-07-25 12:39 a.m., Leon Romanovsky wrote: > > > > On Fri, Jul 23, 2021 at 10:20:50AM -0600, Logan Gunthorpe wrote: > > > > > > > > > > > > > > > > > > > > On 2021-07-23 5:32 a.m., Leon Romanovsky wrote: > > > > > > On Fri, Jul 23, 2021 at 07:06:41PM +0800, Dongdong Liu wrote: > > > > > > > PCIe spec 5.0 r1.0 section 2.2.6.2 says that if an Endpoint supports > > > > > > > sending Requests to other Endpoints (as opposed to host memory), the > > > > > > > Endpoint must not send 10-Bit Tag Requests to another given Endpoint > > > > > > > unless an implementation-specific mechanism determines that the Endpoint > > > > > > > supports 10-Bit Tag Completer capability. Add "pci=disable_10bit_tag=" > > > > > > > parameter to disable 10-Bit Tag Requester if the peer device does not > > > > > > > support the 10-Bit Tag Completer. This will make P2P traffic safe. > > > > > > > > > > > > > > Signed-off-by: Dongdong Liu <liudongdong3@xxxxxxxxxx> > > > > > > > --- > > > > > > > Documentation/admin-guide/kernel-parameters.txt | 7 ++++ > > > > > > > drivers/pci/pci.c | 56 +++++++++++++++++++++++++ > > > > > > > drivers/pci/pci.h | 1 + > > > > > > > drivers/pci/pcie/portdrv_pci.c | 13 +++--- > > > > > > > drivers/pci/probe.c | 9 ++-- > > > > > > > 5 files changed, 78 insertions(+), 8 deletions(-) > > > > > > > > > > > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > > > > > > index bdb2200..c2c4585 100644 > > > > > > > --- a/Documentation/admin-guide/kernel-parameters.txt > > > > > > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > > > > > > @@ -4019,6 +4019,13 @@ > > > > > > > bridges without forcing it upstream. Note: > > > > > > > this removes isolation between devices and > > > > > > > may put more devices in an IOMMU group. > > > > > > > + disable_10bit_tag=<pci_dev>[; ...] > > > > > > > + Specify one or more PCI devices (in the format > > > > > > > + specified above) separated by semicolons. > > > > > > > + Disable 10-Bit Tag Requester if the peer > > > > > > > + device does not support the 10-Bit Tag > > > > > > > + Completer.This will make P2P traffic safe. > > > > > > > > > > > > I can't imagine more awkward user experience than such kernel parameter. > > > > > > > > > > > > As a user, I will need to boot the system, hope for the best that system > > > > > > works, write down all PCI device numbers, guess which one doesn't work > > > > > > properly, update grub with new command line argument and reboot the > > > > > > system. Any HW change and this dance should be repeated. > > > > > > > > > > There are already two such PCI parameters with this pattern and they are > > > > > not that awkward. pci_dev may be specified with either vendor/device IDS > > > > > or with a path of BDFs (which protects against renumbering). > > > > > > > > Unfortunately, in the real world, BDF is not so stable. It changes with > > > > addition of new hardware, BIOS upgrades and even broken servers. > > > > > > That's why it supports using a *path* of BDFs which tends not to catch > > > the wrong device if the topology changes. > > > > > > > Vendor/device IDs doesn't work if you have multiple devices of same > > > > vendor in the system. > > > > > > Yes, but it's fine for some use cases. That's why there's a range of > > > options. > > > > The thing is that you are adding PCI parameter that is applicable to everyone. > > > > We probably see different usage models for this feature. In my world, users > > have thousands of servers that runs 24x7, with VMs on top, some of them perform > > FW upgrades without stopping anything. The idea that you can reboot such server > > any time, simply doesn't exist. > > > > So if I need to enable/disable this feature for one of the VFs, I will be stuck. > > > > > > > > > > > > > > > This flag is only useful in P2PDMA traffic, and if the user attempts > > > > > such a transfer, it prints a warning (see the next patch) with the exact > > > > > parameter that needs to be added to the command line. > > > > > > > > Dongdong citied PCI spec and it was very clear - don't enable this > > > > feature unless you clearly know that it is safe to enable. This is > > > > completely opposite to the proposal here - always enable and disable > > > > if something is printed to the dmesg. > > > > > > Quoting from patch 4: > > > > > > "For platforms where the RC supports 10-Bit Tag Completer capability, > > > it is highly recommended for platform firmware or operating software > > > that configures PCIe hierarchies to Set the 10-Bit Tag Requester Enable > > > bit automatically in Endpoints with 10-Bit Tag Requester capability. > > > This enables the important class of 10-Bit Tag capable adapters that > > > send Memory Read Requests only to host memory." > > > > > > Notice the last sentence. It's saying that devices who only talk to host > > > memory should have 10-bit tags enabled. In the kernel we call devices > > > that talk to things besides host memory "P2PDMA". So the spec is saying > > > not to enable 10bit tags for devices participating in P2PDMA. The kernel > > > needs a way to allow users to do that. The kernel parameter only stops > > > the feature from being enabled for a specific device, and the only > > > use-case is P2PDMA which is not that common and requires the user to be > > > aware of their topology. So I really don't think this is that big a problem. > > > > I'm not question the feature and the need of configuration. My concern > > is just *how* this feature is configured. > > > > > > > > > > > > > > > This has worked well for disable_acs_redir and was used for > > > > > resource_alignment before that for quite some time. So save a better > > > > > suggestion I think this is more than acceptable. > > > > > > > > I don't know about other parameters and their history, but we are not in > > > > 90s anymore and addition of modules parameters (for the PCI it is kernel > > > > cmdline arguments) are better to be changed to some configuration tool/sysfs. > > > > > > The problem was that the ACS bits had to be set before the kernel > > > enumerated the devices. The IOMMU code simply was not able to support > > > dynamic adjustments to its groups. I assume changing 10bit tags > > > dynamically is similarly tricky -- but if it's not then, yes a sysfs > > > interface in addition to the kernel parameter would be a good idea. > > > > I think that it is doable with combination of drivers_autoprobe disable > > and some sysfs knob to enable/disable this feature before driver bind. > > > > It should be very similar to that we did for the dynamic MSI-X, see > > /sys/bus/pci/devices/.../sriov_vf_msix_count > > Many thanks for your suggestion. > > Seems a sysfs could be work ok, but need to make sure 10-Bit Tag Requester > to be set before binding the device driver as > PCIe spec 5.0 section 7.5.3.16 Device Control 2 Register > 10-Bit Tag Requester Enable says that > If software changes the value of this bit while the Function > has outstanding Non-Posted Requests, the result is undefined. This is where drivers_autoprobe will help. Thanks > > Thanks, > Dongdong > > > > Thanks > > > > > > > > Logan > > . > >
