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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: mkvOjt3NwDJF7CEJBTqSQ+lRvozEQiM/UTv4xCxBFjALCFzQh+S7ecFiWqyFTT9RFWYGt8+B33xN2eCCxtwAD36lhVHTi38/6G/KmBfVg+n+WAfedM4147XrUUg883IDY+C3sUxqAKBOvNRUqJt+h7wutguUY0F93Wx7lO3CZOKy6BHx1WLhbDCzA7UXiw76+ROGoQaXdNfeYS4qVgHup/RDTc9eQ3RTv60YS1i6e/UswroaLLMtiGR6e4wkfhEq+2fjCH4KIDF+mKMU1XJFl6QnWoFENAkLHx4xNW6Pp52EHiAkxlJOavc5SzMoTaMqJueT939KZOw8GnMB2mfhggtXTdgJ2Tt/bVBfOMEJ6oFM6xDrFjj1sPx7E/LewFUqeeIjAZrRi6u98eVGKoKajlP4jUxqXOiYMqfL22Jw69WYpw3UdxAkKiIdV2FzMqWI X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Feb 2026 17:02:40.5958 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: c72932e9-123b-498d-c4ad-08de68c6334f X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM4PEPF00027A69.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: GVXPR08MB10406 Hi Catalin, > Hi Levi, > > On Tue, Feb 10, 2026 at 09:54:49AM +0000, Yeoreum Yun wrote: > > > On Fri, Feb 06, 2026 at 06:42:19PM +0000, Catalin Marinas wrote: > > > > On Wed, Jan 21, 2026 at 07:06:17PM +0000, Yeoreum Yun wrote: > > > > > +#ifdef CONFIG_ARM64_LSUI > > > > > +static bool has_lsui(const struct arm64_cpu_capabilities *entry, int scope) > > > > > +{ > > > > > + if (!has_cpuid_feature(entry, scope)) > > > > > + return false; > > > > > + > > > > > + /* > > > > > + * A CPU that supports LSUI should also support FEAT_PAN, > > > > > + * so that SW_PAN handling is not required. > > > > > + */ > > > > > + if (WARN_ON(!__system_matches_cap(ARM64_HAS_PAN))) > > > > > + return false; > > > > > + > > > > > + return true; > > > > > +} > > > > > +#endif > > > > > > > > I still find this artificial dependency a bit strange. Maybe one doesn't > > > > want any PAN at all (software or hardware) and won't get LSUI either > > > > (it's unlikely but possible). > > > > We have the uaccess_ttbr0_*() calls already for !LSUI, so maybe > > > > structuring the macros in a way that they also take effect with LSUI. > > > > For futex, we could add some new functions like uaccess_enable_futex() > > > > which wouldn't do anything if LSUI is enabled with hw PAN. > > > > > > Hmm, I forgot that we removed CONFIG_ARM64_PAN for 7.0, so it makes it > > > harder to disable. Give it a try but if the macros too complicated, we > > > can live with the additional check in has_lsui(). > > > > > > However, for completeness, we need to check the equivalent of > > > !system_uses_ttbr0_pan() but probing early, something like: > > > > > > if (IS_ENABLED(CONFIG_ARM64_SW_TTBR0_PAN) && > > > !__system_matches_cap(ARM64_HAS_PAN)) { > > > pr_info_once("TTBR0 PAN incompatible with FEAT_LSUI; disabling FEAT_LSUI"); > > > return false; > > > } > > > > TBH, I'm not sure whether it's a artifical dependency or not. > > AFAIK, FEAT_PAN is mandatory from Armv8.1 and the FEAT_LSUI seems to > > implements based on the present of "FEAT_PAN". > > > > So, for a hardware which doesn't have FEAT_PAN but has FEAT_LSUI > > sounds like "wrong" hardware and I'm not sure whether it's right > > to enable FEAT_LSUI in this case. > > In principle we shouldn't have such hardware but, as Will pointed out, > we might have such combination due to other reasons like virtualisation, > id reg override. > > It's not that FEAT_LSUI requires FEAT_PAN but rather that the way you > implemented it, the FEAT_LSUI futex code is incompatible with SW_PAN > because you no longer call uaccess_enable_privileged(). So I suggested a > small tweak above to make this more obvious. I would also remove the > WARN_ON, or at least make it WARN_ON_ONCE() if you still want the stack > dump. > > However... > > > SW_PAN case is the same problem. Since If system uses SW_PAN, > > that means this hardware doesn't have a "FEAT_PAN" > > So this question seems to ultimately boil down to whether > > it is appropriate to allow the use of FEAT_LSUI > > even when FEAT_PAN is not supported. > > > > That's why I think the purpose of "has_lsui()" is not for artifical > > dependency but to disable for unlike case which have !FEAT_PAN and FEAT_LSUI > > and IMHO it's enough to check only check with "ARM64_HAS_PAN" instead of > > making a new function like uaccess_enable_futex(). > > Why not keep uaccess_enable_privileged() in > arch_futex_atomic_op_inuser() and cmpxchg for all cases and make it a > no-op if FEAT_LSUI is implemented together with FEAT_PAN? This is because I had a assumption FEAT_PAN must be present when FEAT_LSUI is presented and this was not considering the virtualisation case. and FEAT_PAN is present uaccess_ttbr0_enable() becomes nop and following feedback you gave - https://lore.kernel.org/all/aJ9oIes7LLF3Nsp1@arm.com/ and the reason you mention last, It doesn't need to call mte_enable_tco(). That's why I thought it doesn't need to call uaccess_enable_privileged(). But for a compatibility with SW_PAN, I think we can put only uaccess_ttbr0_enable() in arch_futex_atomic_op_inuser() and cmpxchg simply instead of adding a new APIs uaccess_enable_futex() and by doing this I think has_lsui() can be removed with its WRAN. Am I missing something? > A quick grep shows a recent addition in __lse_swap_desc() (and the llsc equivalent) > but this one can also use CAST with FEAT_LSUI. Thanks. I'll apply this with FEAT_LSUI in next round. > > BTW, with the removal of uaccess_enable_privileged(), we now get MTE tag > checks for the futex operations. I think that's good as it matches the > other uaccess ops, though it's a slight ABI change. If we want to > preserve the old behaviour, we definitely need > uaccess_enable_privileged() that only does mte_enable_tco(). I think we don't need to preserve the old behaviour. so we can skip mte_enable_tco() in case of FEAT_LSUI is presented. Thanks. -- Sincerely, Yeoreum Yun