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Tue, 7 Oct 2025 20:20:40 +0000 Message-ID: Date: Tue, 7 Oct 2025 13:20:37 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 01/15] x86/cpu: Enumerate the LASS feature bits To: Dave Hansen , "Edgecombe, Rick P" , "dave.hansen@linux.intel.com" CC: "corbet@lwn.net" , "ardb@kernel.org" , "david.laight.linux@gmail.com" , "luto@kernel.org" , "jpoimboe@kernel.org" , "andrew.cooper3@citrix.com" , "Luck, Tony" , "alexander.shishkin@linux.intel.com" , "kas@kernel.org" , "seanjc@google.com" , "rdunlap@infradead.org" , "dwmw@amazon.co.uk" , "vegard.nossum@oracle.com" , "xin@zytor.com" , "linux-kernel@vger.kernel.org" , "linux-doc@vger.kernel.org" , "kees@kernel.org" , "hpa@zytor.com" , "peterz@infradead.org" , "linux-efi@vger.kernel.org" , "geert@linux-m68k.org" , "tglx@linutronix.de" , "mingo@redhat.com" , "bp@alien8.de" , "x86@kernel.org" References: <20251007065119.148605-1-sohil.mehta@intel.com> <20251007065119.148605-2-sohil.mehta@intel.com> <47fb7efd89698f46a305ca446d0e4471d1f24fbb.camel@intel.com> <5d95d421-1413-46de-a578-c2a0e44e3aa1@intel.com> Content-Language: en-US From: Sohil Mehta In-Reply-To: <5d95d421-1413-46de-a578-c2a0e44e3aa1@intel.com> Content-Type: text/plain; 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Thus, this series relies on SMAP > being present. > The spec says, "A supervisor-mode data access causes a LASS violation if it would access a linear address of which bit 63 is 0, supervisor-mode access protection is enabled (by setting CR4.SMAP), and either RFLAGS.AC = 0 or the access is an implicit supervisor-mode access." One could argue that the LASS hardware enforcement of the kernel data accesses *depends* on SMAP being enabled. > Actually, it might be worth breaking this dependency hunk out into its > own patch, just so there's a nice clean place to discuss this. Sure, we can talk about the above wording in the spec, as well as the STAC/CLAC dependency in a separate patch. I included some information in the cover letter to explain that: When there are valid reasons for the kernel to access memory in the user half, it can temporarily suspend LASS enforcement by toggling the RFLAGS.AC bit. Most of these cases are already covered today through the stac()/clac() pairs, which avoid SMAP violations. However, there are kernel usages, such as text poking, that access mappings (!_PAGE_USER) in the lower half of the address space. LASS-specific AC bit toggling is added for these cases.