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Fri, 27 Feb 2026 11:04:50 -0500 (EST) Date: Fri, 27 Feb 2026 09:04:49 -0700 From: Alex Williamson To: Jason Gunthorpe Cc: David Matlack , Bjorn Helgaas , Adithya Jayachandran , Alexander Graf , Alex Mastro , Alistair Popple , Andrew Morton , Ankit Agrawal , Bjorn Helgaas , Chris Li , David Rientjes , Jacob Pan , Jonathan Corbet , Josh Hilke , Kevin Tian , kexec@lists.infradead.org, kvm@vger.kernel.org, Leon Romanovsky , Leon Romanovsky , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, Lukas Wunner , =?UTF-8?B?TWlj?= =?UTF-8?B?aGHFgg==?= Winiarski , Mike Rapoport , Parav Pandit , Pasha Tatashin , Pranjal Shrivastava , Pratyush Yadav , Raghavendra Rao Ananta , Rodrigo Vivi , Saeed Mahameed , Samiullah Khawaja , Shuah Khan , Thomas =?UTF-8?B?SGVsbHN0csO2bQ==?= , Tomita Moeko , Vipin Sharma , Vivek Kasireddy , William Tu , Yi Liu , Zhu Yanjun , alex@shazbot.org Subject: Re: [PATCH v2 03/22] PCI: Inherit bus numbers from previous kernel during Live Update Message-ID: <20260227090449.2a23d06d@shazbot.org> In-Reply-To: <20260226144057.GA5933@nvidia.com> References: <20260129212510.967611-4-dmatlack@google.com> <20260225224746.GA3714478@bhelgaas> <20260226144057.GA5933@nvidia.com> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.51; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 26 Feb 2026 10:40:57 -0400 Jason Gunthorpe wrote: > On Wed, Feb 25, 2026 at 11:20:10PM +0000, David Matlack wrote: > > On 2026-02-25 04:47 PM, Bjorn Helgaas wrote: > > > On Thu, Jan 29, 2026 at 09:24:50PM +0000, David Matlack wrote: > > > > Inherit bus numbers from the previous kernel during a Live Update when > > > > one or more PCI devices are being preserved. This is necessary so that > > > > preserved devices can DMA through the IOMMU during a Live Update > > > > (changing bus numbers would break IOMMU translation). > > > > > > I think changing bus numbers would break DMA regardless of whether an > > > IOMMU is involved. Completions carrying the data for DMA reads are > > > routed back to the Requester ID of the read. > > > > Ahh, makes sense. I'll clarify the commit message in the next > > version. > > More broadly you can't shouldn't the fabric topology while Memory > Enable is active. > > Renumbering or readdressing the fabric requires disabling and flushing > any memory transactions. > > From that reasoning it is clearer that you can't do that if the device > is expected to hitlesslly continue performing memory operations. > > That may be a clearer long term basis for describing the requirements > here. Not only fabric topology, but also routing. ACS overrides on the command line would need to be enforced between the original and kexec kernel such that IOMMU groups are deterministic. Thanks, Alex