Date:   Tue, 27 Jul 2021 22:30:40 +0800
From:   Dongdong Liu <liudongdong3@...wei.com>
To:     Leon Romanovsky <leon@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>
CC:     <helgaas@...nel.org>, <hch@...radead.org>, <kw@...ux.com>,
        <linux-pci@...r.kernel.org>, <rajur@...lsio.com>,
        <hverkuil-cisco@...all.nl>, <linux-media@...r.kernel.org>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH V6 7/8] PCI: Add "pci=disable_10bit_tag=" parameter for
 peer-to-peer support



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@...wei.com>
>>>>>> ---
>>>>>>  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.

Thanks,
Dongdong
>
> Thanks
>
>>
>> Logan
> .
>

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