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Mon, 07 Apr 2025 23:37:46 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHnqBBqwWUYhtFhCnTMpC42RqocSQyCJuLXXFnIrjI7SQP3rPRugb4tcWmMuJB3JDe5p/oXtw== X-Received: by 2002:a17:90b:514e:b0:304:ec28:4437 with SMTP id 98e67ed59e1d1-306a4b70e72mr18510574a91.22.1744094266139; Mon, 07 Apr 2025 23:37:46 -0700 (PDT) Received: from [192.168.68.55] ([180.233.125.65]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-305983bce79sm10080504a91.37.2025.04.07.23.37.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 07 Apr 2025 23:37:45 -0700 (PDT) Message-ID: <7fa4269b-a20c-4cfc-b6e7-e70214ec6366@redhat.com> Date: Tue, 8 Apr 2025 16:37:36 +1000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 34/45] kvm: rme: Hide KVM_CAP_READONLY_MEM for realm guests To: Steven Price , kvm@vger.kernel.org, kvmarm@lists.linux.dev Cc: Catalin Marinas , Marc Zyngier , Will Deacon , James Morse , Oliver Upton , Suzuki K Poulose , Zenghui Yu , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Joey Gouly , Alexandru Elisei , Christoffer Dall , Fuad Tabba , linux-coco@lists.linux.dev, Ganapatrao Kulkarni , Shanker Donthineni , Alper Gun , "Aneesh Kumar K . V" References: <20250213161426.102987-1-steven.price@arm.com> <20250213161426.102987-35-steven.price@arm.com> <32a09a27-f131-44dd-8959-abb63b2089a8@redhat.com> Content-Language: en-US From: Gavin Shan In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 4/8/25 2:34 AM, Steven Price wrote: > On 04/03/2025 11:51, Gavin Shan wrote: >> On 2/14/25 2:14 AM, Steven Price wrote: >>> For protected memory read only isn't supported. While it may be possible >>> to support read only for unprotected memory, this isn't supported at the >>> present time. >>> >>> Signed-off-by: Steven Price >>> --- >>>   arch/arm64/kvm/arm.c | 2 +- >>>   1 file changed, 1 insertion(+), 1 deletion(-) >>> >> >> It's worthy to explain why KVM_CAP_READONLY_MEM isn't supported and its >> negative impact. It's something to be done in the future if I'm correct. > > I'll add to the commit message: > > Note that this does mean that e.g. ROM (or flash) data cannot be > emulated correctly by the VMM. > Please also to mention this if you agree: At present, there is no exposed APIs from RMM allowing to specifying stage-2 page-table entry's permission. read-only regions for ROM and flash have to be backed up by read-write stage-2 page-table entries. It's going to rely on the stage-1 page-table to have the proper permissions for those read-only regions. >> From QEMU's perspective, all ROM data, which is populated by it, can >> be written. It conflicts to the natural limit: all ROM data should be >> read-only. > > Yes this is my understanding of the main impact. I'm not sure how useful > (shared) ROM/flash emulation is. It can certainly be added in the future > if needed. Protected read-only memory I don't believe is useful - the > only sane response I can see from a write fault in that case is killing > the guest. > Yes, VMM is still able to write to those regions even they're read-only since they're emulated. For misbehaving guest where those regions are also mapped as read-write, the data resident in those regions can be corrupted by guest. It's not the expected output. Since RMM doesn't have exposed APIs allowing to specify page-table entry's permissions, meaning all entries have read-write permissions, we have to give read-write permission to those read-only regions for now. In long run, it's something to be fixed, starting from RMM. Thanks, Gavin > Thanks, > Steve > >> QEMU >> ==== >> rom_add_blob >>   rom_set_mr >>     memory_region_set_readonly >>       memory_region_transaction_commit >>         kvm_region_commit >>           kvm_set_phys_mem >>             kvm_mem_flags                                    // flag >> KVM_MEM_READONLY is missed >>             kvm_set_user_memory_region >>               kvm_vm_ioctl(KVM_SET_USER_MEMORY_REGION2) >> >> non-secure host >> =============== >> rec_exit_sync_dabt >>   kvm_handle_guest_abort >>     user_mem_abort >>       __kvm_faultin_pfn                       // writable == true >>         realm_map_ipa >>           WARN_ON(!(prot & KVM_PGTABLE_PROT_W) >> >> non-secure host >> =============== >> kvm_realm_enable_cap(KVM_CAP_ARM_RME_POPULATE_REALM) >>   kvm_populate_realm >>     __kvm_faultin_pfn                      // writable == true >>       realm_create_protected_data_page >> >>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c >>> index 1f3674e95f03..0f1d65f87e2b 100644 >>> --- a/arch/arm64/kvm/arm.c >>> +++ b/arch/arm64/kvm/arm.c >>> @@ -348,7 +348,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, >>> long ext) >>>       case KVM_CAP_ONE_REG: >>>       case KVM_CAP_ARM_PSCI: >>>       case KVM_CAP_ARM_PSCI_0_2: >>> -    case KVM_CAP_READONLY_MEM: >>>       case KVM_CAP_MP_STATE: >>>       case KVM_CAP_IMMEDIATE_EXIT: >>>       case KVM_CAP_VCPU_EVENTS: >>> @@ -362,6 +361,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, >>> long ext) >>>       case KVM_CAP_COUNTER_OFFSET: >>>           r = 1; >>>           break; >>> +    case KVM_CAP_READONLY_MEM: >>>       case KVM_CAP_SET_GUEST_DEBUG: >>>           r = !kvm_is_realm(kvm); >>>           break; >> >> Thanks, >> Gavin >> >