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Zyngier , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "will@kernel.org" , "ryan.roberts@arm.com" , "shahuang@redhat.com" , "lpieralisi@kernel.org" , "david@redhat.com" , Aniket Agashe , Neo Jia , Kirti Wankhede , "Tarun Gupta (SW-GPU)" , Vikram Sethi , Andy Currid , Alistair Popple , John Hubbard , Dan Williams , Zhi Wang , Matt Ochs , Uday Dhoke , Dheeraj Nigam , Krishnakant Jaju , "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" , "ddutile@redhat.com" , "tabba@google.com" , "qperret@google.com" , "kvmarm@lists.linux.dev" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags Message-ID: <20250429181926.GE2260709@nvidia.com> References: <20250423120243.GD1648741@nvidia.com> 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nPS2SD1uFTWpYnt9EnTzPouFhY3ifj3t3JWk2FHq8UmJ4RuWN/dNFcmExOMVxqTlHbs07ARtufJ606QmWbzLLc+nVWEw7U5meW7AM8h2Cr3vLy06a4Qw+BiBsAqJn336L7xML+eZcXLWpZ8/hy+0JV0FEEkVRMYsNgcLZYScbRSkGJiRDjYnEAJPopqIQURJBU5FzUEg0c1XFc94xNTscryEeXJVrotCxB8J/bI2Wc2R4WUe7WzBByTmrglnwrXmbHMTTL8dBPSM5iA4yXVXpCJd9K3Xy4qgyYaZEjPnWPW+6ycpVw3H8CBIBBdlRT3G9JTQ/PJoznAxt6a1C0xVbWorLZ83MnJDNG4BFMgrBu24YiF7tSk0YHuet2oiRsne15HAHVE1VHJk+4Z+dhMNsZCX03BG2LiNr8VnZpuLkGttFIYs/8E7oqM0Z6Pzfv2WWxTlw X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Apr 29, 2025 at 07:09:42PM +0100, Catalin Marinas wrote: > On Tue, Apr 29, 2025 at 01:44:30PM -0300, Jason Gunthorpe wrote: > > On Tue, Apr 29, 2025 at 05:03:18PM +0100, Catalin Marinas wrote: > > > On Tue, Apr 29, 2025 at 11:14:37AM -0300, Jason Gunthorpe wrote: > > > > On Tue, Apr 29, 2025 at 02:27:02PM +0100, Catalin Marinas wrote: > > > > > BTW, we should reject exec mappings as well (they probably fail for S1 > > > > > VFIO since set_pte_at() will try to do cache maintenance). > > > > > > > > To be clear the S2 should leave the mapping as execute allowed > > > > though. Only the VM knows how it will use this memory and VM's do > > > > actually execute out of the cachable PFNMAP VMA today. The VM will set > > > > any execute deny/allow on its S1 table according to how it uses the > > > > memory. > > > > > > If S2 is executable, wouldn't KVM try to invalidate the I-cache and it > > > won't have an alias to do this? Unless it doesn't end up in > > > stage2_map_walker_try_leaf() or the walk has been flagged as skipping > > > the CMO. > > > > Okay, that does seem to have been overlooked a bit. The answer I got > > back is: > > > > Cachable PFNMAP is also relying on ARM64_HAS_CACHE_DIC also, simlar to > > how S2FWB allows KVM to avoid flushing the D cache, that CPU cap > > allows KVM to avoid flushing the icache and turns icache_inval_pou() > > into a NOP. > > Another CAP for executable PFNMAP then? IDK, either that or a more general cap 'support PFNMAP VMAs'? > I feel like this is a different > use-case (e.g. more like general purpose CXL attached memory) than the > GPU one. The GPUs we have today pretty much pretend to be CXL attached memory so they can and do execute from it. > Unless FWB implies CTR_EL0.DIC (AFAIK, it doesn't) we may be > restricting some CPUs. Yes, it will further narrow the CPUs down. However, we just did this discussion for BBML2 + SMMUv3 SVA. I think the same argument holds. If someone is crazy enough to build a CPU with CXLish support and uses an old core without DIC, IDC and S2FWB then they are going to have a bunch of work to fix the SW to support it. Right now we know of no system that exists like this.. Jason