Re: [PATCH v5 10/11] PCI: liveupdate: Do not disable bus mastering on preserved devices during kexec

[Bjorn Helgaas]

On Tue, May 12, 2026 at 06:48:45PM +0000, David Matlack wrote:
> Do not disable bus mastering on outgoing preserved devices during
> pci_device_shutdown() for kexec.
> 
> Preserved devices must be allowed to perform memory transactions during
> a Live Update to minimize downtime and ensure continuous operation.
> Clearing the bus mastering bit would prevent these devices from issuing
> any memory requests while the new kernel boots.

s/minimize downtime and//

Ensuring continuous operation is the critical piece here.  Minimizing
downtime is always good but IMO it's not actually a reason for this
change.

I guess what this is probably saying is that the continuous operation
and Live Update are what minimizes downtime, and preserving bus
mastering is a piece of that.

> Because bridges upstream of preserved endpoint devices are also
> automatically preserved, this change also avoids clearing bus mastering
> on them. This is critical because clearing bus mastering on an upstream
> bridge prevents the bridge from forwarding memory requests upstream (i.e.
> it would prevent the endpoint device from accessing system RAM and doing
> peer-to-peer transactions with devices not downstream of the bridge).