Re: [PATCH] PCI/MSI: Re-add checks for skip masking MSI-X on Xen PV

From: Bjorn Helgaas <helgaas@kernel.org>
To: Josef Johansson <josef@oderland.se>
Cc: linux-pci@vger.kernel.org,
	xen-devel <xen-devel@lists.xenproject.org>,
	Jason Andryuk <jandryuk@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Marc Zyngier <maz@kernel.org>
Subject: Re: [PATCH] PCI/MSI: Re-add checks for skip masking MSI-X on Xen PV
Date: Tue, 19 Oct 2021 15:29:06 -0500	[thread overview]
Message-ID: <20211019202906.GA2397931@bhelgaas> (raw)
In-Reply-To: <906d370d-5023-e856-4ead-f0b499aded53@oderland.se>

On Tue, Oct 19, 2021 at 10:15:05PM +0200, Josef Johansson wrote:
> On 10/19/21 21:57, Bjorn Helgaas wrote:
> > On Mon, Oct 18, 2021 at 08:22:32AM +0200, Josef Johansson wrote:

> I'll make an effort to do a better commit log. Thanks for reviewing the
> entry!
> 
> What is the time frame for v5.15?

Soon.  v5.15-rc7 should happen Oct 24, final release likely Oct 31.
Ideally a fix would be in before the 24th.

> >> This patch solves a major issue with booting 5.15-rc1 under Xen
> >> with amdgpu drivers. Specifically Lenovo P14s Gen 1, AMD 4750U.
> >>
> >> The softlock below takes about ~2-3 minutes to release, and will
> >> lock again if I switch between X and console, when staying in
> >> X I can do things without it softlock.
> >
> > I don't actually see a softlock mentioned below; am I missing
> > something?  It's nice to include a couple lines of dmesg log to help
> > people connect the issue with the fix, but most of the below is not
> > relevant and can be easily found from the Link: tag.  Also, some of
> > the lines seem to be wrapped.  They're more useful when not wrapped
> > because grep can find them.
> 
> Sorry for my lack of words here. I used deadlock when I first
> described it, but since it was released after a while, I thought
> softlock would be more fitting.

I looked up the WARN_ON at amdgpu_dm.c:8630 but I didn't see anything
related to deadlock or soft lockup or any kind of delay:

  WARN_ON(acrtc_attach->pflip_status != AMDGPU_FLIP_NONE);

DRM folks might have an idea.

> I'll dig a bit to try to get a better dmesg around the stacktrace. Sorry
> about the wrapping,
> 
> I'm trying hard to keep those 80 chars ;-)

I appreciate that.  Commit logs should be 75 chars because git log
indents by 4.  Quotes of dmesg and the like should be unwrapped.

> >> I should also note that my original problem with *flip done timeout*
> >> inside drm during suspend/resume is not solved, but with this patch
> >> at least the kernel boots properly.
> >>
> >> The kernel is much more stable not running inside Xen, and much 
> >> more stable running pci=nomsi (under Xen). Are we missing something
> >> more regarding masking?
> > It does sound like something else is broken as well, but I have no
> > idea what that would be.
> 
> Should I take a stab at describing the issue better at hand or
> should we focus on getting this specific patch out of the way?

Unless the other issues seem related, we should dispose of this one
by itself ASAP.

> I have 'solved' my current problems a bit by kernel flags right now,
> but I would be happy to share my story so far.

Description of your workaround would be quite useful.  Probably not to
*me*, but to people who know the area (I'd start with the DRM folks).

> >> The error that occurs is:
> >>
> >> kernel: ------------[ cut here ]------------
> >> kernel: WARNING: CPU: 6 PID: 3754 at
> >> drivers/gpu/drm/amd/amdgpu/../display/amdgp
> >> u_dm/amdgpu_dm.c:8630 amdgpu_dm_commit_planes+0x9b4/0x9c0 [amdgpu]