Date:   Tue, 27 Jul 2021 22:00:35 +0800
From:   Dongdong Liu <liudongdong3@...wei.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Leon Romanovsky <leon@...nel.org>
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 2021/7/26 23:48, 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.
>
>>>
>>> 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.
>
>>>
>>> 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.
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.

So 10-Bit Tag Requester Enable should be set before probe the device 
driver.

Thanks,
Dongdong
>
> Logan
> .
>

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