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[2003:d8:2f2c:5500:988:23f9:faa0:7232]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-454a1705ed5sm47240875e9.2.2025.07.04.07.13.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 04 Jul 2025 07:13:24 -0700 (PDT) Message-ID: <07e0fddd-76ba-4905-95f4-b98063215866@redhat.com> Date: Fri, 4 Jul 2025 16:13:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 5/6] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags To: Jason Gunthorpe , ankita@nvidia.com Cc: maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, ryan.roberts@arm.com, shahuang@redhat.com, lpieralisi@kernel.org, ddutile@redhat.com, seanjc@google.com, aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, kjaju@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com, zhiw@nvidia.com, mochs@nvidia.com, udhoke@nvidia.com, dnigam@nvidia.com, 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 References: <20250621042111.3992-1-ankita@nvidia.com> <20250621042111.3992-6-ankita@nvidia.com> <20250704140431.GH1410929@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250704_071331_109106_014B1ED1 X-CRM114-Status: GOOD ( 28.68 ) 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 04.07.25 16:04, Jason Gunthorpe wrote: > On Sat, Jun 21, 2025 at 04:21:10AM +0000, ankita@nvidia.com wrote: >> --- a/arch/arm64/kvm/mmu.c >> +++ b/arch/arm64/kvm/mmu.c >> @@ -1681,18 +1681,53 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, >> if (is_error_noslot_pfn(pfn)) >> return -EFAULT; >> >> + /* >> + * Check if this is non-struct page memory PFN, and cannot support >> + * CMOs. It could potentially be unsafe to access as cachable. >> + */ >> if (vm_flags & (VM_PFNMAP | VM_MIXEDMAP) && !pfn_is_map_memory(pfn)) { >> /* >> - * If the page was identified as device early by looking at >> - * the VMA flags, vma_pagesize is already representing the >> - * largest quantity we can map. If instead it was mapped >> - * via __kvm_faultin_pfn(), vma_pagesize is set to PAGE_SIZE >> - * and must not be upgraded. >> - * >> - * In both cases, we don't let transparent_hugepage_adjust() >> - * change things at the last minute. >> + * COW VM_PFNMAP is possible when doing a MAP_PRIVATE >> + * /dev/mem mapping on systems that allow such mapping. >> + * Reject such case. >> */ >> - s2_force_noncacheable = true; >> + if (is_cow_mapping(vm_flags)) >> + return -EINVAL; > > I still would like an explanation why we need to block this. > > COW PFNMAP is like MIXEDMAP, you end up with a VMA where there is a > mixture of MMIO and normal pages. Arguably you are supposed to use > vm_normal_page() not pfn_is_map_memory(), but that seems difficult for > KVM. > > Given we exclude the cachable case with the pfn_is_map_memory() we > know this is the non-struct page memory already, so why do we need to > block the COW? > > I think the basic rule we are going for is that within the VMA the > non-normal/special PTE have to follow the vma->vm_pgprot while the > normal pages have to be cachable. > > So if we find a normal page (ie pfn_is_map_memory()) then we know it > is cachable and s2_force_noncacheable = false. Otherwise we use the > vm_pgprot to decide if the special PTE is cachable. > > David can you think of any reason to have this is_cow_mapping() test? I think with that reasoning, it should be fine to drop it. I think, the COW test made sense when we were talking about limiting it to VM_PFNMAP only and simplifying by dropping other checks. Then, it would have identified that something is certainly not "normal" memory. -- Cheers, David / dhildenb