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: <841e1793-4635-4f06-b0c9-37530215c08b@suse.com>
Date: Wed, 29 Jan 2025 14:52:33 +0100
From: Jan Beulich <jbeulich@...e.com>
To: Bjorn Helgaas <helgaas@...nel.org>
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,
 Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
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 14:32, Bjorn Helgaas wrote:
> On Wed, Jan 29, 2025 at 04:47:28AM +0100, Marek Marczykowski-Górecki wrote:
>> On Tue, Jan 28, 2025 at 09:40:18PM -0600, Bjorn Helgaas wrote:
>>> I guess the code at [2] is running in user mode and uses Linux
>>> syscalls for config access?  Is it straceable?
>>
>> Nope, it's running as the hypervisor and mediates Linux's access to the
>> hardware. In fact, Linux PV kernel (which dom0 is by default under Xen)
>> is running in ring 3...
>>
>> But I can add some more logging there.
> 
> So I guess the hypervisor performs the config access on behalf of the
> Linux PV kernel?  Obviously Linux thinks CRS/RRS SV is enabled, but I
> suppose all the lspci output is similarly based on whatever the
> hypervisor does, so how do we know the actual hardware config?

The hypervisor only intercepts config space writes; reads, particularly
when going via mmcfg, ought to be unaffected, and hence what lspci shows
should be "the actual hardware config".

Jan