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[159.2.239.150]) by smtp.gmail.com with ESMTPSA id af79cd13be357-9404722bccbsm30639685a.8.2026.10.23.07.29.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Oct 2026 07:29:10 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1xKGGb-0000000GjTu-3HLf; Fri, 23 Oct 2026 11:29:09 -0300 Date: Fri, 23 Oct 2026 11:29:09 -0300 From: Jason Gunthorpe To: Robin Murphy Cc: Samiullah Khawaja , Joerg Roedel , Will Deacon , Kevin Tian , Alex Williamson , iommu@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Pasha Tatashin , David Matlack , Lu Baolu , Pranjal Shrivastava Subject: Re: [RFC PATCH 3/3] iommu/arm-smmu-v3: Add support for hitless replace of S1 domains Message-ID: <20261023142909.GG3977851@ziepe.ca> References: <20260925011308.3381953-1-skhawaja@google.com> <20260925011308.3381953-4-skhawaja@google.com> <3d1eb92f-3447-49f0-a210-fd4fa015304b@arm.com> <20261023134900.GB3977851@ziepe.ca> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Oct 23, 2026 at 03:04:23PM +0100, Robin Murphy wrote: > On 23/10/2026 2:49 pm, Jason Gunthorpe wrote: > > On Fri, Oct 23, 2026 at 02:43:28PM +0100, Robin Murphy wrote: > > > > @@ -3664,8 +3726,8 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev, > > > > struct arm_smmu_cd target_cd; > > > > arm_smmu_make_s1_cd(&target_cd, master, smmu_domain); > > > > - arm_smmu_write_cd_entry(master, IOMMU_NO_PASID, cdptr, > > > > - &target_cd); > > > > + arm_smmu_replace_cd_entry(master, IOMMU_NO_PASID, cdptr, > > > > + &target_cd); > > > > > > No, we cannot blindly do this for arbitrary ->attach_dev calls - the > > > majority of S1->S1 transitions are going to be moving back and forth between > > > the default domain and user domains, where there is zero expectation, let > > > alone guarantee, that the translations would be equivalent and thus not > > > liable to lead to TLB conflict aborts or amalgamation hell. > > > > Heh, yeah this probably only works properly with BBML2 or something so > > the temporary ASID doesn't explode. > > This is fundamental VMSA behaviour - see R_FVQCK in the Arm ARM. BBML only > applies to the one specific case of identical VA->PA translations with > identical attributes _only_ differing in the leaf level at which they are > described. Sigh, I thought ARM got past that 'tlb conflicts catch fire'. So, yeah we need a flag that says "I pinky swear these are identical translations". And it has to be a kernel only flag since we can't let userspace trigger this. Jason