linux-kernel - Re: Config space access to Mediatek MT7922 doesn't work after device reset in Xen PV dom0 (regression, Linux 6.12)

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Message-ID: <Z5oWq4YgMgwWvl2G@mail-itl>
Date: Wed, 29 Jan 2025 12:53:15 +0100
From: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
To: Jan Beulich <jbeulich@...e.com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Jürgen Groß <jgross@...e.com>,
	Roger Pau Monné <roger.pau@...rix.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	xen-devel <xen-devel@...ts.xenproject.org>,
	linux-kernel@...r.kernel.org, regressions@...ts.linux.dev,
	Felix Fietkau <nbd@....name>, Lorenzo Bianconi <lorenzo@...nel.org>,
	Ryder Lee <ryder.lee@...iatek.com>, linux-pci@...r.kernel.org,
	Bjorn Helgaas <helgaas@...nel.org>
Subject: Re: Config space access to Mediatek MT7922 doesn't work after device
 reset in Xen PV dom0 (regression, Linux 6.12)

On Wed, Jan 29, 2025 at 10:17:20AM +0100, Jan Beulich wrote:
> On 29.01.2025 04:22, Marek Marczykowski-Górecki wrote:
> > On Tue, Jan 28, 2025 at 09:03:15PM -0600, Bjorn Helgaas wrote:
> >> The report claims the problem only happens with Xen.  I'm not a Xen
> >> person, and I don't know how to find the relevant config accessors.
> >> The snippets of kernel messages I see at [1] all mention pciback, so
> >> that's my only clue of where to look.  Bottom line, I have no idea
> >> what the config accessor path is, and maybe we could learn something
> >> by looking at whatever it is.
> > 
> > AFAIK there are no separate config accessors under Xen dom0, the default
> > ones are used. xen-pcifront takes over PCI config space access (and few
> > more) only in a domU (and only for PV), when PCI passthrough is used.
> > Here, it didn't went that far...
> > 
> > But then, Xen may intercept such access [2]. If I read it right, it
> > should allow all access (is_hardware_domain(dom0)==true, and also the
> > device is not on ro_map - otherwise reset wouldn't work at all).
> 
> The other day you mentioned (on Matrix I think) that you observe mmcfg
> not being used on that system. Am I misremembering? (Since the capability
> where the control bit lives is an extended one, that capability would
> neither be read nor modified when mmcfg is unavailable.)

Yes, but later (once dom0 starts) it switched back to mmcfg. Now I see
this:
(XEN) PCI: MCFG configuration 0: base e0000000 segment 0000 buses 00 - ff
(XEN) PCI: Using MCFG for segment 0000 bus 00-ff

Another thing I noticed in the bug report - the reporter says warm
reboot from 6.11 (where it works) to 6.12 avoids the issue (not sure
about further reboots). Cold boot directly to 6.12 results in this buggy
behavior.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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