Re: [PATCH V3 03/10] PCI/TPH: Add pci=notph to prevent use of TPH

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Jul 24, 2024 at 03:05:59PM -0500, Wei Huang wrote:
> 
> 
> On 7/23/24 17:41, Bjorn Helgaas wrote:
> > On Wed, Jul 17, 2024 at 03:55:04PM -0500, Wei Huang wrote:
> >> TLP headers with incorrect steering tags (e.g. caused by buggy driver)
> >> can potentially cause issues when the system hardware consumes the tags.
> > 
> > Hmm.  What kind of issues?  Crash?  Data corruption?  Poor
> > performance?
> 
> Not crash or functionality errors. Usually it is QoS related because of
> resource competition. AMD has

Looks like you had more to say here?

I *assume* that both the PH hint and the Steering Tags are only
*hints* and there's no excuse for hardware to corrupt anything (e.g.,
by omitting cache maintenance) even if the hint turns out to be wrong.
If that's the case, I assume "can potentially cause issues" really
just means "might lead to lower performance".  That's what I want to
clarify and confirm.

> >> Provide a kernel option, with related helper functions, to completely
> >> prevent TPH from being enabled.
> > 
> > Also would be nice to have a hint about the difference between "notph"
> > and "nostmode".  Maybe that goes in the "nostmode" patch?  I'm not
> > super clear on all the differences here.
> 
> I can combine them. Here is the combination and it meaning based on TPH
> Control Register values:
> 
> Requestor Enable | ST Mode | Meaning
> ---------------------------------------------------------------
> 00               | xx      | TPH disabled (i.e. notph)
> 01               | 00      | TPH enabled, NO ST Mode (i.e. nostmode)
> 01 or 11         | 01      | Interrupt Vector mode
> 01 or 11         | 10      | Device specific mode
> 
> If you have any other thoughts on how to approach these modes, please
> let me know.

IIRC, there's no interface in this series that reall does anything
with TPH per se; drivers would only use the ST-related things.

If that's the case, maybe "pci=notph" isn't needed yet.

Bjorn