Re: [PATCH V7 7/9] PCI/sysfs: Add a 10-Bit Tag sysfs file

[Bjorn Helgaas <helgaas@kernel.org>]

On Sat, Aug 07, 2021 at 03:01:56PM +0800, Dongdong Liu wrote:
> 
> On 2021/8/5 23:31, Bjorn Helgaas wrote:
> > On Thu, Aug 05, 2021 at 04:37:39PM +0800, Dongdong Liu wrote:
> > > 
> > > 
> > > On 2021/8/5 7:49, Bjorn Helgaas wrote:
> > > > On Wed, Aug 04, 2021 at 09:47:06PM +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 a 10bit_tag sysfs file,
> > > > > write 0 to disable 10-Bit Tag Requester when the driver does not bind
> > > > > the device if the peer device does not support the 10-Bit Tag Completer.
> > > > > This will make P2P traffic safe. the 10bit_tag file content indicate
> > > > > current 10-Bit Tag Requester Enable status.
> > > > > 
> > > > > Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
> > > > > ---
> > > > >  Documentation/ABI/testing/sysfs-bus-pci | 16 +++++++-
> > > > >  drivers/pci/pci-sysfs.c                 | 69 +++++++++++++++++++++++++++++++++
> > > > >  2 files changed, 84 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> > > > > index 793cbb7..0e0c97d 100644
> > > > > --- a/Documentation/ABI/testing/sysfs-bus-pci
> > > > > +++ b/Documentation/ABI/testing/sysfs-bus-pci
> > > > > @@ -139,7 +139,7 @@ Description:
> > > > >  		binary file containing the Vital Product Data for the
> > > > >  		device.  It should follow the VPD format defined in
> > > > >  		PCI Specification 2.1 or 2.2, but users should consider
> > > > > -		that some devices may have incorrectly formatted data.
> > > > > +		that some devices may have incorrectly formatted data.
> > > > >  		If the underlying VPD has a writable section then the
> > > > >  		corresponding section of this file will be writable.
> > > > > 
> > > > > @@ -407,3 +407,17 @@ Description:
> > > > > 
> > > > >  		The file is writable if the PF is bound to a driver that
> > > > >  		implements ->sriov_set_msix_vec_count().
> > > > > +
> > > > > +What:		/sys/bus/pci/devices/.../10bit_tag
> > > > > +Date:		August 2021
> > > > > +Contact:	Dongdong Liu <liudongdong3@huawei.com>
> > > > > +Description:
> > > > > +		If a PCI device support 10-Bit Tag Requester, will create the
> > > > > +		10bit_tag sysfs file. The file is readable, the value
> > > > > +		indicate current 10-Bit Tag Requester Enable.
> > > > > +		1 - enabled, 0 - disabled.
> > > > > +
> > > > > +		The file is also writeable, the value only accept by write 0
> > > > > +		to disable 10-Bit Tag Requester when the driver does not bind
> > > > > +		the deivce. The typical use case is for p2pdma when the peer
> > > > > +		device does not support 10-BIT Tag Completer.
> > 
> > > > > +static ssize_t pci_10bit_tag_store(struct device *dev,
> > > > > +				   struct device_attribute *attr,
> > > > > +				   const char *buf, size_t count)
> > > > > +{
> > > > > +	struct pci_dev *pdev = to_pci_dev(dev);
> > > > > +	bool enable;
> > > > > +
> > > > > +	if (kstrtobool(buf, &enable) < 0)
> > > > > +		return -EINVAL;
> > > > > +
> > > > > +	if (enable != false )
> > > > > +		return -EINVAL;
> > > > 
> > > > Is this the same as "if (enable)"?
> > > Yes, Will fix.
> > 
> > I actually don't like the one-way nature of this.  When the hierarchy
> > supports 10-bit tags, we automatically enable them during enumeration.
> > 
> > Then we provide this sysfs file, but it can only *disable* 10-bit
> > tags.  There's no way to re-enable them except by rebooting (or using
> > setpci, I guess).
> > 
> > Why can't we allow *enabling* them here if they're supported in this
> > hierarchy?
> Yes, we can also provide this sysfs to enable 10-bit tag for EP devices
> when the hierarchy supports 10-bit tags.
> 
> I do not want to provide sysfs to enable/disable 10-bit tag for RP
> devices as I can not tell current if the the Function has outstanding
> Non-Posted Requests, may need to unbind all the EP drivers under the
> RP, and current seems no scenario need to do this. This will make things
> more complex.

You mean "no scenario requires disabling 10-bit tags and then
re-enabling them"?  That may be true, but I'm still hesitant to
provide a switch than can only be reversed by rebooting.

This is similar to the issue Leon raised that it's not practical to
reboot machines.  Maybe we accept a one-way switch if the sole purpose
is to work around a hardware defect.  Or maybe a kernel parameter that
disables 10-bit tags completely is the defect mitigation.  I think we
probably need such a parameter in case a defect prevents us from
booting far enough to use the sysfs switch.