linux-kernel - Re: [PATCH] PCI: quirk for preventing bus reset on TI C667X

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Date:   Fri, 29 Jan 2021 17:49:46 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Antti Järvinen <antti.jarvinen@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alex Williamson <alex.williamson@...hat.com>,
        Murali Karicheri <m-karicheri2@...com>,
        Kishon Vijay Abraham I <kishon@...com>
Subject: Re: [PATCH] PCI: quirk for preventing bus reset on TI C667X

On Tue, Jan 26, 2021 at 01:22:18PM +0200, Antti Järvinen wrote:
> On 22.1.2021 1.55, Bjorn Helgaas wrote:> 
> 
> > It looks like we would probably be trying a Secondary Bus Reset using
> > the bridge leading to the C667X.  Can you confirm? 
> 
> Yes, this is my understanding too.
> 
> > Wonder if you
> > could try doing what pci_reset_secondary_bus() does by hand:
> > 
> 
> I tried this by hand. It looks that result is same as through VFIO.
> 
> # cat sbr.sh
> BRIDGE=10:00.0
> C667X=11:00.0
> 
> setpci -s$C667X VENDOR_ID.w
> 
> VAL=$(setpci -s$BRIDGE BRIDGE_CONTROL.w)
> echo $VAL
> setpci -s$BRIDGE BRIDGE_CONTROL.w=$(($VAL | 0x40))
> sleep 1
> setpci -s$BRIDGE BRIDGE_CONTROL.w=$VAL
> sleep 1
> setpci -s$C667X VENDOR_ID.w=0
> setpci -s$C667X VENDOR_ID.w
> 
> 
> # ./sbr.sh
> 104c
> 0003
> ffff
> 
> 
> >   # BRIDGE=...                              # PCI address, e.g., 00:1c.0
> >   # C667X=...
> >   # setpci -s$C667X VENDOR_ID.w
> >   # setpci -s$BRIDGE BRIDGE_CONTROL.w       # prints "val"
> >   # setpci -s$BRIDGE BRIDGE_CONTROL.w=      # val | 0x40 (set SBR)
> >   # sleep 1
> >   # setpci -s$BRIDGE BRIDGE_CONTROL.w=      # val (clear SBR)
> >   # sleep 1
> >   # setpci -s$C667X VENDOR_ID.w=0
> >   # setpci -s$C667X VENDOR_ID.w
> > 
> > If we use this quirk and avoid the reset, I assume that means
> > assigning the device to VMs with VFIO will leak state between VMs?
> 
> I think this is true.

Alex, is there some warning about situations like this where a device
may leak state between VMs?

There's nothing in quirk_no_bus_reset() itself where we set
PCI_DEV_FLAGS_NO_BUS_RESET, and nothing in pci_parent_bus_reset() or
pci_dev_reset_slot_function() where we test it, but I didn't chase
into VFIO.

Seems important enough that we might want to mention it at least once
and maybe even every time we try to reset the device.  I hope the leak
isn't completely silent.

Bjorn