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Wed, 31 Jul 2024 12:36:40 +0000 Message-ID: Date: Wed, 31 Jul 2024 18:06:31 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] arm/kvm: add support for MTE From: Ganapatrao Kulkarni To: =?UTF-8?Q?Alex_Benn=C3=A9e?= , Peter Maydell Cc: qemu-arm@nongnu.org, qemu-devel@nongnu.org, richard.henderson@linaro.org, darren@os.amperecomputing.com References: <20240709060448.251881-1-gankulkarni@os.amperecomputing.com> <0c171de4-a8ea-4859-b78c-272244267bb3@os.amperecomputing.com> <8734ns1p3y.fsf@draig.linaro.org> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MAXP287CA0024.INDP287.PROD.OUTLOOK.COM (2603:1096:a00:49::32) To SJ2PR01MB8101.prod.exchangelabs.com (2603:10b6:a03:4f6::10) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR01MB8101:EE_|BL3PR01MB6995:EE_ X-MS-Office365-Filtering-Correlation-Id: 8089514e-a6aa-4cf5-d4b1-08dcb15d6d3f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014; 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envelope-from=gankulkarni@os.amperecomputing.com; helo=DM5PR21CU001.outbound.protection.outlook.com X-Spam_score_int: -19 X-Spam_score: -2.0 X-Spam_bar: -- X-Spam_report: (-2.0 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 29-07-2024 04:10 pm, Ganapatrao Kulkarni wrote: > > > On 29-07-2024 03:44 pm, Alex Bennée wrote: >> Ganapatrao Kulkarni writes: >> >>> Hi Peter, >>> >>> >>> [Apologies for the delayed response] >>> >>> On 16-07-2024 09:15 pm, Peter Maydell wrote: >>>> On Tue, 9 Jul 2024 at 07:05, Ganapatrao Kulkarni >>>> wrote: >>>>> >>>>> Extend the 'mte' property for the virt machine to cover KVM as >>>>> well. For KVM, we don't allocate tag memory, but instead enable >>>>> the capability. >>>>> >>>>> If MTE has been enabled, we need to disable migration, as we do not >>>>> yet have a way to migrate the tags as well. Therefore, MTE will stay >>>>> off with KVM unless requested explicitly. >>>>> >>>>> This patch is rework of commit >>>>> b320e21c48ce64853904bea6631c0158cc2ef227 >>>>> which broke TCG since it made the TCG -cpu max >>>>> report the presence of MTE to the guest even if the board hadn't >>>>> enabled MTE by wiring up the tag RAM. This meant that if the guest >>>>> then tried to use MTE QEMU would segfault accessing the >>>>> non-existent tag RAM. >>>>> >>>>> Signed-off-by: Cornelia Huck >>>>> Signed-off-by: Ganapatrao Kulkarni >>>>> >>>>> --- >>>> In target/arm/cpu.c:arm_cpu_realizefn() there is this code: >>>>       if (cpu_isar_feature(aa64_mte, cpu)) { >>>>           /* >>>>            * The architectural range of GM blocksize is 2-6, however >>>> qemu >>>>            * doesn't support blocksize of 2 (see HELPER(ldgm)). >>>>            */ >>>>           if (tcg_enabled()) { >>>>               assert(cpu->gm_blocksize >= 3 && cpu->gm_blocksize <= 6); >>>>           } >>>> #ifndef CONFIG_USER_ONLY >>>>           /* >>>>            * If we do not have tag-memory provided by the machine, >>>>            * reduce MTE support to instructions enabled at EL0. >>>>            * This matches Cortex-A710 BROADCASTMTE input being LOW. >>>>            */ >>>>           if (cpu->tag_memory == NULL) { >>>>               cpu->isar.id_aa64pfr1 = >>>>                   FIELD_DP64(cpu->isar.id_aa64pfr1, ID_AA64PFR1, >>>> MTE, 1); >>>>           } >>>> #endif >>>>       } >>>> With this patch, for KVM we will end up going through the >>>> "squash ID_AA64PFR1_EL1.MTE to 1" codepath, because KVM doesn't >>>> set cpu->tag_memory and this is still using that as its check. >>>> >>> >>> I looked at this function and it seems we are not entering this >>> function for KVM boot. I do see -DCONFIG_USER_ONLY added to make >>> files. >>> My bad, please ignore my previous/above comment. I did not hit this issue since cpu_isar_feature(aa64_mte, cpu) is returning zero/false on my ARM64 platform. Then I dumped the register id_aa64pfr1 at QEMU(qemu-system-aarch64) as well in Linux(vanilla 6.10) kernel(for ioctl KVM_GET_ONE_REG) and to my surprise, in qemu the value is 0x21 however the value at kernel is 0x321(expected value). Root-caused and it is due to, kernel is hiding[1] the MTE bits of ID_AA64PFR1_EL1 register from user/qemu. Need to send the kernel patch upstream to revert it, otherwise this check in qemu is dummy. [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v6.11-rc1&id=2ac638fc5724f011f8ba1b425667c5592e1571ce >>> Also Linux kernel wont detect/enable MTE until unless the >>> ID_AA64PFR1_EL1.MTE value is 2(b0010) and above. >>> >>>> More generally, how does the enabling of the MTE KVM cap >>>> interact with the ID_AA64PFR1_EL1 value that we read from >>>> the host in kvm_arm_get_host_cpu_features() ? We care that we >>>> have the right ID register values because we use ID field >>>> checks to determine whether the vcpu has a feature or not, >>>> even in the KVM case. >>>> Since Cornelia first wrote the patch this is based on, we've >>>> landed gdbstub support for MTE (so gdb can find out which >>>> addresses in the memory map have tags and read and write >>>> those tags). So I think the KVM MTE support now also needs to >>>> handle that. (See aarch64_cpu_register_gdb_commands() in >>>> target/arm/gdbstub64.c.) >>> >>> Ok sure, I will go through this file to add/update MTE part >> >> So to be clear the current MTE gdbstub support is linux-user only. >> Gustavo has a series on the list that adds the system emulation part: >> >>    Message-Id: <20240722160709.1677430-1-gustavo.romero@linaro.org> >>    Date: Mon, 22 Jul 2024 16:07:05 +0000 >>    Subject: [PATCH 0/4] gdbstub: Add support for MTE in system mode >>    From: Gustavo Romero >> >> which of course is focused on TCG. But if the KVM guests sync to the same >> registers to cpregs I think most stuff should just work. However the >> current code uses the TCG only: >> >>    allocation_tag_mem_probe >> >> which I guess needs a KVM equivalent to query the tag memory? > > Ok, thanks for the heads-up!. > Thanks, Ganapat