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Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1uZ7m4-00000005KhJ-2YcX; Tue, 08 Jul 2025 12:50:16 +0000 Received: from dfw.source.kernel.org ([139.178.84.217]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1uZ7jj-00000005KKh-2txc for linux-arm-kernel@lists.infradead.org; Tue, 08 Jul 2025 12:47:52 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by dfw.source.kernel.org (Postfix) with ESMTP id E48215C60D3; Tue, 8 Jul 2025 12:47:50 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id DEA48C4CEED; Tue, 8 Jul 2025 12:47:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1751978870; bh=osuOgrPS2qSqQveywb0omF1aib+vgM1aAsa1ykg2SH4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=IAMlhLI7cory+R2Bv4G4I7BkDERGpi3VYc+VJGMY+ZmNiGrhtDEs5lgtn+8wiu4u6 EZFwzSY16+e5Ji4D7Z6CjUvZ/aXOfk0tPqg+X7zkamPyn+7COaeDs9tzZNxW6e1Bde RDRJRPH4YlXqHDgCmiM5y9tWAtKbqTUdHFW/6cSunfJLTNzYsSBxUUuFjipdJmxqcw K7hhKc+761noYebQwzzUpbXXXpHiwdELTf7fK7AeLVsfnlClHKizMH3s0RzYNTa7lh cOMY4Vu2GYsVOri9ahMntePMKOUl0lTQF5na6epLFdZ7pqzS6A+Z0ZkjhA69R2BzK6 PfXrRaTRRXXBg== Date: Tue, 8 Jul 2025 13:47:38 +0100 From: Will Deacon To: Jason Gunthorpe Cc: David Hildenbrand , Ankit Agrawal , "maz@kernel.org" , "oliver.upton@linux.dev" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "ryan.roberts@arm.com" , "shahuang@redhat.com" , "lpieralisi@kernel.org" , "ddutile@redhat.com" , "seanjc@google.com" , Aniket Agashe , Neo Jia , Kirti Wankhede , Krishnakant Jaju , "Tarun Gupta (SW-GPU)" , Vikram Sethi , Andy Currid , Alistair Popple , John Hubbard , Dan Williams , Zhi Wang , Matt Ochs , Uday Dhoke , Dheeraj Nigam , "alex.williamson@redhat.com" , "sebastianene@google.com" , "coltonlewis@google.com" , "kevin.tian@intel.com" , "yi.l.liu@intel.com" , "ardb@kernel.org" , "akpm@linux-foundation.org" , "gshan@redhat.com" , "linux-mm@kvack.org" , "tabba@google.com" , "qperret@google.com" , "kvmarm@lists.linux.dev" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "maobibo@loongson.cn" , "pbonzini@redhat.com" Subject: Re: [PATCH v9 3/6] KVM: arm64: Block cacheable PFNMAP mapping Message-ID: References: <20250621042111.3992-1-ankita@nvidia.com> <20250621042111.3992-4-ankita@nvidia.com> <20250630122501.GQ167785@nvidia.com> <4b06b163-e1ce-4c20-b878-4593bc86bf53@redhat.com> <20250704164750.GO1410929@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250704164750.GO1410929@nvidia.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250708_054751_816829_6656C024 X-CRM114-Status: GOOD ( 32.26 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Jul 04, 2025 at 01:47:50PM -0300, Jason Gunthorpe wrote: > On Fri, Jul 04, 2025 at 05:04:17PM +0100, Will Deacon wrote: > > On Fri, Jul 04, 2025 at 02:21:32PM +0200, David Hildenbrand wrote: > > > On 30.06.25 14:25, Jason Gunthorpe wrote: > > > > On Mon, Jun 30, 2025 at 01:56:43AM +0000, Ankit Agrawal wrote: > > > > > > Sorry for the drive-by comment, but I was looking at this old series from > > > > > > Paolo (look at the cover letter and patch 5): > > > > > > > > > > > > https://lore.kernel.org/r/20250109133817.314401-1-pbonzini@redhat.com > > > > > > > > > > > > in which he points out that the arm64 get_vma_page_shift() function > > > > > > incorrectly assumes that a VM_PFNMAP VMA is physically contiguous, which > > > > > > may not be the case if a driver calls remap_pfn_range() to mess around > > > > > > with mappings within the VMA. I think that implies that the optimisation > > > > > > in 2aa53d68cee6 ("KVM: arm64: Try stage2 block mapping for host device > > > > > > MMIO") is unsound. > > > > > > > > > > Hm yeah, that does seem problematic. Perhaps we need a new > > > > > vma flag that could help the driver communicate to the KVM that the > > > > > mapping is contiguous and it can go ahead with the optimization? > > > > > E.g. something similar to VM_ALLOW_ANY_UNCACHED. > > > > > > > > I think Paolo has the right direction - remove any attempts by KVM to > > > > expand contiguity, it should only copy the primary's PTEs and rely on > > > > the primary to discover contiguity. No new flags. > > > > > > 100% > > > > The part I don't understand, however, is that I can't see an MMU notifier > > anywhere on the successful remap_pfn_range() path. So if a driver is > > using that interface to change the mapping properties of a VM_PFNMAP VMA, > > how do we ensure that the guest doesn't use whatever stale mappings it's > > faulted in previously? Did I just miss something? > > Generally mmu notifiers are for invalidation, not used when > establishing new mappings. > > It is not legal to use remap_pfn_range() to replace already mapped > PTEs. It can only be used during a fop mmap callback to establish the > first mapping during VMA creation. Thus there can be no present > mapping cached in a secondary and no need to invalidate. Aha, thanks, I had completely missed that. I thought remap_pfn_range() was quietly unmapping any existing mapping it ran into, but that's not the case at all. Cheers, Will