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: <4d7ed713-68b5-4e9c-8952-002d21d662d6@suse.com>
Date: Wed, 29 Jan 2025 13:49:20 +0100
From: Jan Beulich <jbeulich@...e.com>
To: Marek Marczykowski-Górecki
 <marmarek@...isiblethingslab.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 29.01.2025 12:53, Marek Marczykowski-Górecki wrote:
> 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.

Makes things yet more odd, imo.

Jan