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Fri, 11 Mar 2022 05:43:02 +0000 Received: from BY5PR12MB4289.namprd12.prod.outlook.com ([fe80::4843:222b:2955:543f]) by BY5PR12MB4289.namprd12.prod.outlook.com ([fe80::4843:222b:2955:543f%8]) with mapi id 15.20.5038.029; Fri, 11 Mar 2022 05:43:02 +0000 Message-ID: Date: Fri, 11 Mar 2022 11:12:43 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging Content-Language: en-US To: David Laight , 'Dave Hansen' , "linux-kernel@vger.kernel.org" Cc: "linux-mm@kvack.org" , "x86@kernel.org" , "kirill.shutemov@linux.intel.com" , "tglx@linutronix.de" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "catalin.marinas@arm.com" , "will@kernel.org" , "shuah@kernel.org" , "oleg@redhat.com" , "ananth.narayan@amd.com" References: <20220310111545.10852-1-bharata@amd.com> <699fb763ac054833bc8c29c9814c63b2@AcuMS.aculab.com> <286efdb9-7dbf-82f3-b172-29c575a3791b@intel.com> From: Bharata B Rao In-Reply-To: Content-Type: text/plain; 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dkim=pass header.d=amd.com header.s=selector1 header.b=XZBwMIxE; dmarc=pass (policy=quarantine) header.from=amd.com; spf=pass (imf07.hostedemail.com: domain of bharata@amd.com designates 40.107.244.68 as permitted sender) smtp.mailfrom=bharata@amd.com X-Stat-Signature: hbjkm8cymrxp6uwfsdoqqst9fhbcztdd X-HE-Tag: 1646977385-897004 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000007, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 3/10/2022 10:49 PM, David Laight wrote: > From: Dave Hansen >> Sent: 10 March 2022 16:46 >> >> On 3/10/22 06:32, David Laight wrote: >>>> UAI allows software to store a tag in the upper 7 bits of a logical >>>> address [63:57]. When enabled, the processor will suppress the >>>> traditional canonical address checks on the addresses. More information >>>> about UAI can be found in section 5.10 of 'AMD64 Architecture >>>> Programmer's Manual, Vol 2: System Programming' which is available from >>>> >>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.kernel.org%2Fattachment.cgi%3Fid%3D300549&data=04%7C01%7Cbharata%40amd.com%7Ca1de24223931481b3fcb08da02ba2e6f%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637825295938946622%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=HijEAUq172r8YwkcCuhvl99Vk5BwE6iSROXcSQXmJHk%3D&reserved=0 >>> Is that really allowing bit 63 to be used? >>> That is normally the user-kernel bit. >>> I can't help feeling that will just badly break things. >> >> Yeah, this does seem worrisome. The LAM approach[1] retains >> canonicality checking for bit 63. > > Actually it is rather worse than 'worrisome'. > Allowing the user all address upto the base of the valid > kernel addresses (probably tags to 3e, but not 3f) > means that you can't use a fast address check in access_ok(). > You are forced to use the strict test that 32bit kernels use. >From what I see, there is a single implementation of access_ok() in arch/x86/asm/include/uaccess.h that does check if the user address+size exceeds the limit. Guess I am missing something, but can you please point me to the fast implementation(that benefits from bit 63 being user/kernel address disambiguation bit) and the strict checking in 32bit kernels that are you are referring to? Also I wonder here why ARM64 TBI which also uses the full upper byte (including bit 63) for storing tag/metadata doesn't suffer from this same problem? Regards, Bharata.