linux-kernel - Re: [RFC PATCH 0/3] PCI: Use Configuration RRS to wait for device ready

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240828214218.GA40398@bhelgaas>
Date: Wed, 28 Aug 2024 16:42:18 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Mario Limonciello <mario.limonciello@....com>
Cc: linux-pci@...r.kernel.org,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	"Maciej W . Rozycki" <macro@...am.me.uk>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Lukas Wunner <lukas@...ner.de>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Duc Dang <ducdang@...gle.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	"Li, Gary" <Gary.Li@....com>
Subject: Re: [RFC PATCH 0/3] PCI: Use Configuration RRS to wait for device
 ready

On Wed, Aug 28, 2024 at 04:24:01PM -0500, Mario Limonciello wrote:
> On 8/27/2024 18:48, Bjorn Helgaas wrote:
> > From: Bjorn Helgaas <bhelgaas@...gle.com>
> > 
> > After a device reset, pci_dev_wait() waits for a device to become
> > completely ready by polling the PCI_COMMAND register.  The spec envisions
> > that software would instead poll for the device to stop responding to
> > config reads with Completions with Request Retry Status (RRS).
> > 
> > Polling PCI_COMMAND leads to hardware retries that are invisible to
> > software and the backoff between software retries doesn't work correctly.
> > 
> > Root Ports are not required to support the Configuration RRS Software
> > Visibility feature that prevents hardware retries and makes the RRS
> > Completions visible to software, so this series only uses it when available
> > and falls back to PCI_COMMAND polling when it's not.
> > 
> > This is completely untested and posted for comments.
> > 
> > Bjorn Helgaas (3):
> >    PCI: Wait for device readiness with Configuration RRS
> >    PCI: aardvark: Correct Configuration RRS checking
> >    PCI: Rename CRS Completion Status to RRS
> > 
> >   drivers/bcma/driver_pci_host.c             | 10 ++--
> >   drivers/pci/controller/dwc/pcie-tegra194.c | 18 +++---
> >   drivers/pci/controller/pci-aardvark.c      | 64 +++++++++++-----------
> >   drivers/pci/controller/pci-xgene.c         |  6 +-
> >   drivers/pci/controller/pcie-iproc.c        | 18 +++---
> >   drivers/pci/pci-bridge-emul.c              |  4 +-
> >   drivers/pci/pci.c                          | 41 +++++++++-----
> >   drivers/pci/pci.h                          | 11 +++-
> >   drivers/pci/probe.c                        | 33 +++++------
> >   include/linux/bcma/bcma_driver_pci.h       |  2 +-
> >   include/linux/pci.h                        |  1 +
> >   include/uapi/linux/pci_regs.h              |  6 +-
> >   12 files changed, 117 insertions(+), 97 deletions(-)
> 
> Although this looks like a useful series, I'm sorry to say but this doesn't
> solve the issue that Gary and I raised.  We double checked today and found
> that reading the vendor ID works just fine at this time.

Thanks for testing that.

> I think that we're still better off polling PCI_PM_CTRL to "wait" for D0
> after the state change from D3cold.

Is there some spec justification for polling PCI_PM_CTRL?  I'm dubious
about doing that just because "it works" in this situation, unless we
have some better understanding about *why* it works and whether all
devices are supposed to work that way.

Bjorn