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[34.124.168.21]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2d5bb499d4bsm5258655ad.4.2026.08.17.05.53.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 Aug 2026 05:53:42 -0700 (PDT) Date: Mon, 17 Aug 2026 12:53:36 +0000 From: Pranjal Shrivastava To: Jason Gunthorpe Cc: Dmitry Malkin , Will Deacon , Robin Murphy , Joerg Roedel , Nicolin Chen , "iommu@lists.linux.dev" , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "regressions@lists.linux.dev" , "stable@vger.kernel.org" , NB-Core Team Subject: Re: [RFC PATCH] iommu/arm-smmu-v3: Allow nested attach for PCI bridges without vDEVICE Message-ID: References: <20260814174825.GC510472@nvidia.com> <20260814221103.GE510472@nvidia.com> <20260817114407.GB1175472@nvidia.com> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260817114407.GB1175472@nvidia.com> On Mon, Aug 17, 2026 at 08:44:07AM -0300, Jason Gunthorpe wrote: > On Mon, Aug 17, 2026 at 11:16:17AM +0000, Pranjal Shrivastava wrote: > > But, regarding PCI P2PDMA, you can still enable it with ACS enabled in > > the upstream kernel *IF* your hardware supports TLP forwarding between > > root ports. > > > > When ACS is enabled, all TLPs get forwarded to the host's Root Ports > > instead of getting routed directly through the switch. > > Generally nobody wants to run like that, the performance is > terrible. The reason these systems have a PCI switch is because of > performance of the P2P path. Ack. I was assuming the enablement was the major challenge. For perf we definitly need to hairpin at the switch. > > Jason Thanks, Praan