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Shutemov" To: Dave Hansen Cc: Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Peter Zijlstra , Ard Biesheuvel , "Paul E. McKenney" , Josh Poimboeuf , Xiongwei Song , Xin Li , "Mike Rapoport (IBM)" , Brijesh Singh , Michael Roth , Tony Luck , Alexey Kardashevskiy , Alexander Shishkin , Jonathan Corbet , Sohil Mehta , Ingo Molnar , Pawan Gupta , Daniel Sneddon , Kai Huang , Sandipan Das , Breno Leitao , Rick Edgecombe , Alexei Starovoitov , Hou Tao , Juergen Gross , Vegard Nossum , Kees Cook , Eric Biggers , Jason Gunthorpe , "Masami Hiramatsu (Google)" , Andrew Morton , Luis Chamberlain , Yuntao Wang , Rasmus Villemoes , Christophe Leroy , Tejun Heo , Changbin Du , Huang Shijie , Geert Uytterhoeven , Namhyung Kim , Arnaldo Carvalho de Melo , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCHv6 06/16] efi: Disable LASS around set_virtual_address_map() EFI call Message-ID: References: <20250620135325.3300848-1-kirill.shutemov@linux.intel.com> <20250620135325.3300848-7-kirill.shutemov@linux.intel.com> <5bb6275b-5046-4bda-8c92-434e59cac08f@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5bb6275b-5046-4bda-8c92-434e59cac08f@intel.com> X-Rspam-User: X-Rspamd-Queue-Id: EFDE4C0002 X-Rspamd-Server: rspam10 X-Stat-Signature: n9wswr3hwedsjsaigeg1c1pq5sj3cfwt X-HE-Tag: 1750441838-780701 X-HE-Meta: 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 QIhlxdjC uJ2s/s/Any/3S2iApwzOaMOLuVzAbBR9XX3/8BYd8qIAejHc3oHrvI9Y0nU28Sak+zbOTx6I1WCejX2GsVIoBizox7XJgmrLIg7piTwlSNObd7LWXUoXyesQOnVQrK4HcYgXrt7J4gj0YXe0N2IblgBunM9qiGdjQdkGI+cJ1DC9k8i4ZAQFtnrPdQTkeXb6hPS+kbN9GWwGUhQmNsqRdts1+28vO5kOzOUs7Lc4BYwvBo7uZys2EJcculonCoVG8EyCx6bbcAzuir8M6ePNulrs24r4bYkeqt/BVqdkjBpxnyPetQseHUCvj9w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jun 20, 2025 at 08:55:29AM -0700, Dave Hansen wrote: > On 6/20/25 06:53, Kirill A. Shutemov wrote: > > diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c > > index 463b784499a8..94c335229697 100644 > > --- a/arch/x86/platform/efi/efi.c > > +++ b/arch/x86/platform/efi/efi.c > > @@ -825,11 +825,24 @@ static void __init __efi_enter_virtual_mode(void) > > > > efi_sync_low_kernel_mappings(); > > > > + /* > > + * set_virtual_address_map is the only service located at lower > > + * addresses, so we have to temporarily disable LASS around it. > > + * Note that clearing EFLAGS.AC is not enough for this, the whole > > + * LASS needs to be disabled. > > + */ > > + if (cpu_feature_enabled(X86_FEATURE_LASS)) > > + cr4_clear_bits(X86_CR4_LASS); > > Could we do it like this instead? > > unsigned long lass = cr4_read_shadow() & X86_FEATURE_LASS; > ... > cr4_clear_bits(lass); > > > status = efi_set_virtual_address_map(efi.memmap.desc_size * count, > > efi.memmap.desc_size, > > efi.memmap.desc_version, > > (efi_memory_desc_t *)pa, > > efi_systab_phys); > > + > > + if (cpu_feature_enabled(X86_FEATURE_LASS)) > > + cr4_set_bits(X86_CR4_LASS); > > and: > > cr4_set_bits(lass); > > > if (status != EFI_SUCCESS) { > > pr_err("Unable to switch EFI into virtual mode (status=%lx)!\n", > > status); > > That way, neither the presence of X86_FEATURE_LASS nor the ordering of > setting up X86_CR4_LASS matters. > > Let's say the CPU supports X86_FEATURE_LASS and this code gets called > before the kernel is ready for LASS. It would break as written above. Okay, will rework this. -- Kiryl Shutsemau / Kirill A. Shutemov