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Fri, 7 Nov 2025 20:08:24 +0000 Message-ID: Date: Fri, 7 Nov 2025 12:08:21 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 9/9] x86/cpu: Enable LASS by default during CPU initialization Content-Language: en-US To: "H. Peter Anvin" , Dave Hansen , , Dave Hansen , Thomas Gleixner , Ingo Molnar , Borislav Petkov CC: Jonathan Corbet , Andy Lutomirski , "Josh Poimboeuf" , Peter Zijlstra , "Ard Biesheuvel" , "Kirill A . Shutemov" , Xin Li , David Woodhouse , Sean Christopherson , Rick Edgecombe , "Vegard Nossum" , Andrew Cooper , Randy Dunlap , Geert Uytterhoeven , Kees Cook , Tony Luck , Alexander Shishkin , , , References: <20251029210310.1155449-1-sohil.mehta@intel.com> <20251029210310.1155449-10-sohil.mehta@intel.com> <789ADBB5-F7AC-4B08-B343-F23260FB8FBC@zytor.com> <255724be-a6d8-4aa6-94f9-1e6ffba3a3cc@zytor.com> From: Sohil Mehta In-Reply-To: <255724be-a6d8-4aa6-94f9-1e6ffba3a3cc@zytor.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BY1P220CA0021.NAMP220.PROD.OUTLOOK.COM (2603:10b6:a03:5c3::16) To DS0PR11MB7997.namprd11.prod.outlook.com (2603:10b6:8:125::14) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7997:EE_|SA2PR11MB4937:EE_ X-MS-Office365-Filtering-Correlation-Id: 7b96c425-cabe-4f8b-e8af-08de1e39686a X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|7416014|376014; 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Peter Anvin wrote: >> I asked Sohil to start throwing out all the non-essential bits from this >> series. My thought was that we could get mainline so that LASS itself >> could be enabled, even if it was in a somewhat weird config that a >> distro would probably never do. >> >> After that is merged, we can circle back and decide what to do with the >> remaining bits like CR pinning and vsyscall emulation. I don't think any >> of those bits will change the basic desire to have LASS support in the >> kernel. >> >> Does that sound like a sane approach to everyone? > > XONLY vsyscall emulation should be trivial, though (just look for the magic > RIP values, just like the page fault code does now, too.) The FULL emulation > mode is completely irrelevant these days, so I don't think it matters at all. > Yes, the actual change is quite simple. But along with minor refactoring and updates to code comments and documentation, it ends up being a set of 3 to 4 small patches. > EFI handling is similarly straightforward: just disable CR4.LASS right before > calling EFI, and enable it on return. As long as we are *already* in the > efi_mm context, it is perfectly safe to disable CR4.LASS, because there *is no > user memory mapped at that point*. > At a minimum, We need to defer the initial LASS enabling until we are truly done with the boot services memory, i.e. efi_enter_virtual_mode() has completed (including efi_free_boot_services()). Also, there is preference for deferring LASS enabling until userspace comes up. Again, all of that should be simple enough but ends up adding to the patch count (touching 20 now). It was getting difficult to get the reviews/acks from experts in all the impacted areas. Dave's suggestion of splitting the series makes it easier to merge the base set and get focussed reviews on the missing features. My plan is to promptly follow up with the EFI and Vsyscall support to make LASS enabled by default. > These two things should only be a few lines of code each, and I don't see any > reason to further elaborate them at this time (to handle FULL emulation, or to Yes, there is absolutely no need to support the full vsyscall emulation. > take a LASS trap inside EFI to write a shame-the-vendor message; if we wanted > to do that, it would be better to make that independent of LASS and empty out > efi_mm entirely. > This seems to be the takeaway of PeterZ's interaction with Ard. Though, I agree, warning about misbehaving firmware should probably be done separately and independent of LASS. I'll send out another revision of this series without EFI next week, and follow up with support for EFI and vsyscall soon after.