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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-442fd50b30csm75388245e9.9.2025.05.17.09.59.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 17 May 2025 09:59:33 -0700 (PDT) Date: Sat, 17 May 2025 17:59:32 +0100 From: David Laight To: Ard Biesheuvel Cc: Borislav Petkov , Ard Biesheuvel , Shivank Garg , linux-kernel@vger.kernel.org, x86@kernel.org, Ingo Molnar , Linus Torvalds , Brian Gerst , "Rao, Bharata Bhasker" Subject: Re: [PATCH v3 1/7] x86/cpu: Use a new feature flag for 5 level paging Message-ID: <20250517175932.2cd2d678@pumpkin> In-Reply-To: References: <20250514104242.1275040-9-ardb+git@google.com> <20250514104242.1275040-10-ardb+git@google.com> <20250515131120.GCaCXn-E8zQutUqKLn@fat_crate.local> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 15 May 2025 14:33:50 +0100 Ard Biesheuvel wrote: > On Thu, 15 May 2025 at 14:11, Borislav Petkov wrote: > > > > On Wed, May 14, 2025 at 12:42:44PM +0200, Ard Biesheuvel wrote: > > > From: Ard Biesheuvel > > > > > > Currently, the LA57 CPU feature flag is taken to mean two different > > > things at once: > > > - whether the CPU implements the LA57 extension, and is therefore > > > capable of supporting 5 level paging; > > > - whether 5 level paging is currently in use. > > > > Btw, that gunk: > > > > We had started simplifying the whole 5-level crap: > > > > https://lore.kernel.org/all/20240621164406.256314-1-kirill.shutemov@linux.intel.com/ > > > > Shivank, I hear the performance issues got resolved in the meantime? > > > > It would be interesting to know whether CONFIG_X86_5LEVEL=n deviates > from CONFIG_X86_5LEVEL=y with 'no5lvl' on the command line. If passing > 'no5lvl' makes up for the performance hit, then I don't think the > performance issues should stop us from removing this Kconfig symbol. You might then want a Kconfig option to invert the default for the command line option (and an inverted command line option). David