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Date: Fri, 22 May 2026 13:01:59 -0500
From: Bjorn Helgaas <helgaas@kernel.org>
To: David Matlack <dmatlack@google.com>
Cc: kexec@lists.infradead.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-pci@vger.kernel.org,
	Adithya Jayachandran <ajayachandra@nvidia.com>,
	Alexander Graf <graf@amazon.com>,
	Alex Williamson <alex@shazbot.org>,
	Bjorn Helgaas <bhelgaas@google.com>, Chris Li <chrisl@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Jacob Pan <jacob.pan@linux.microsoft.com>,
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	Josh Hilke <jrhilke@google.com>,
	Leon Romanovsky <leonro@nvidia.com>, Lukas Wunner <lukas@wunner.de>,
	Mike Rapoport <rppt@kernel.org>, Parav Pandit <parav@nvidia.com>,
	Pasha Tatashin <pasha.tatashin@soleen.com>,
	Pranjal Shrivastava <praan@google.com>,
	Pratyush Yadav <pratyush@kernel.org>,
	Saeed Mahameed <saeedm@nvidia.com>,
	Samiullah Khawaja <skhawaja@google.com>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Vipin Sharma <vipinsh@google.com>, William Tu <witu@nvidia.com>,
	Yi Liu <yi.l.liu@intel.com>
Subject: Re: [PATCH v5 10/11] PCI: liveupdate: Do not disable bus mastering
 on preserved devices during kexec
Message-ID: <20260522180159.GA231364@bhelgaas>
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In-Reply-To: <20260512184846.119396-11-dmatlack@google.com>

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).