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Fix cache inconsistency of the identity-mapped region To: Marc Zyngier Cc: Catalin Marinas , Will Deacon , Mark Rutland , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Ard Biesheuvel , Vikram Sethi , Thierry Reding , Anshuman Khandual References: <20220415170504.3781878-1-sdonthineni@nvidia.com> <87y202agz9.wl-maz@kernel.org> From: Shanker R Donthineni Message-ID: <6b08ece9-64c0-9de9-9876-ed2dceee9bb0@nvidia.com> Date: Mon, 18 Apr 2022 07:53:20 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 In-Reply-To: <87y202agz9.wl-maz@kernel.org> Content-Language: en-US X-ClientProxiedBy: SN6PR04CA0097.namprd04.prod.outlook.com (2603:10b6:805:f2::38) To SN6PR12MB2751.namprd12.prod.outlook.com (2603:10b6:805:6c::26) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 50904f86-b91c-4e35-1d3f-08da213a6c53 X-MS-TrafficTypeDiagnostic: DM5PR12MB1323:EE_ X-Microsoft-Antispam-PRVS: 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Fix cache inconsistency of the identity-mapped region To: Marc Zyngier Cc: Catalin Marinas , Will Deacon , Mark Rutland , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Ard Biesheuvel , Vikram Sethi , Thierry Reding , Anshuman Khandual References: <20220415170504.3781878-1-sdonthineni@nvidia.com> <87y202agz9.wl-maz@kernel.org> From: Shanker R Donthineni Message-ID: <6b08ece9-64c0-9de9-9876-ed2dceee9bb0@nvidia.com> Date: Mon, 18 Apr 2022 07:53:20 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 In-Reply-To: <87y202agz9.wl-maz@kernel.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-ClientProxiedBy: SN6PR04CA0097.namprd04.prod.outlook.com (2603:10b6:805:f2::38) To SN6PR12MB2751.namprd12.prod.outlook.com (2603:10b6:805:6c::26) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 50904f86-b91c-4e35-1d3f-08da213a6c53 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X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: MWtWQaVU8gzPuk/knMWlBcyn+65ncH2xWk3nq0xanDL1n+7dDc1icOZPTmGUyFTxma64e4XQ7tFbocX3fMTfAw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR12MB1323 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thanks Marc for your comments. On 4/18/22 4:16 AM, Marc Zyngier wrote: > External email: Use caution opening links or attachments > > > Hi Shanker, > > On Fri, 15 Apr 2022 18:05:03 +0100, > Shanker Donthineni wrote: >> The secondary cores boot is stuck due to data abort while executing the >> instruction 'ldr x8, =__secondary_switched'. The RELA value of this >> instruction was updated by a primary boot core from __relocate_kernel() >> but those memory updates are not visible to CPUs after calling >> switch_to_vhe() causing problem. >> >> The cacheable/shareable attributes of the identity-mapped regions are >> different while CPU executing in EL1 (MMU enabled) and for a short period >> of time in hyp-stub (EL2-MMU disabled). As per the ARM-ARM specification >> (DDI0487G_b), this is not allowed. >> >> G5.10.3 Cache maintenance requirement: >> "If the change affects the cacheability attributes of the area of memory, >> including any change between Write-Through and Write-Back attributes, >> software must ensure that any cached copies of affected locations are >> removed from the caches, typically by cleaning and invalidating the >> locations from the levels of cache that might hold copies of the locations >> affected by the attribute change." >> >> Clean+invalidate the identity-mapped region till PoC before switching to >> VHE world to fix the cache inconsistency. >> >> Problem analysis with disassembly (vmlinux): >> 1) Both __primary_switch() and enter_vhe() are part of the identity region >> 2) RELA entries and enter_vhe() are sharing the same cache line fff800010970480 >> 3) Memory ffff800010970484-ffff800010970498 is updated with EL1-MMU enabled >> 4) CPU fetches intrsuctions of enter_vhe() with EL2-MMU disabled >> - Non-coherent access causing the cache line fff800010970480 drop > Non-coherent? You mean non-cacheable, right? At this stage, we only > have a single CPU, so I'm not sure coherency is the problem here. When > you say 'drop', is that an eviction? Replaced by what? By the previous > version of the cache line, containing the stale value? Yes,non-cacheable. The cache line corresponding to function enter_vhe() was marked with shareable & WB-cache as a result of write to RELA, the same cache line is being fetched with non-shareable & non-cacheable. The eviction is not pushed to PoC and got dropped because of cache-line attributes mismatch.   > It is also unclear to me how the instruction fetches directly > influence what happens *on the other CPUs*. Is this line kept at a > level beyond the PoU? Are we talking of a system cache here? It would > really help if you could describe your cache topology. > >> 5) Secondary core executes 'ldr x8, __secondary_switched' >> - Getting data abort because of the incorrect value at ffff800010970488 > My interpretation of the above is as follows: > > - CPU0 performs the RELA update with the MMU on > > - A switch to EL2 with the MMU off results in the cache line sitting > beyond the PoU and containing the RELA update to be replaced with > the *stale* version (the fetch happening on the i-side). > > - CPU1 (with its MMU on) fetches the stale data from the cache > > Is this correct? Yes, correct, > What is unclear to me is why the eviction occurs. Yes, this is of > course allowed, and the code is wrong for assuming any odd behaviour. > But then, The eviction is happening naturally don't know the time and the exact line of code where eviction is happening.   > why only clean the idmap? Seems only EL2-STUB code is accessing RELA of idmap with MMU disabled. I did 2 other experiments, the issue was not reproducible.  1) CONFIG_RELOCATABLE=n  2) Change the line "ldr x8, =__secondary_switched" to     adr_l   x9, _text     adr_l   x8, __secondary_switched     sub     x8, x8, x9     mov_q   x9, KIMAGE_VADDR     add     x8, x8, x9 > I have the feeling that by this > standard, we should see this a lot more often. Or are we just lucky > that we don't have any other examples of data and instructions sharing > the same cache line and accessed with different cacheability > attributes? I think in all other places either MMU is enabled or instructions/data pushed to the PoC. > Thanks, > > M. > > -- > Without deviation from the norm, progress is not possible.