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([2a02:6b6f:e750:1b00:1cfc:9209:4810:3ae5]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429897e785asm900083f8f.3.2025.10.22.16.58.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 Oct 2025 16:58:18 -0700 (PDT) Message-ID: <797c78f4-1a90-42da-9fed-e87682456a43@gmail.com> Date: Thu, 23 Oct 2025 00:58:15 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/3] x86/mm: Move _PAGE_BIT_NOPTISHADOW from bit 58 to bit 9 Content-Language: en-GB To: Dave Hansen , dwmw@amazon.co.uk, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, ardb@kernel.org, hpa@zytor.com Cc: x86@kernel.org, apopple@nvidia.com, thuth@redhat.com, nik.borisov@suse.com, kas@kernel.org, linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org, kernel-team@meta.com, Michael van der Westhuizen , Tobias Fleig References: <20251022220755.1026144-1-usamaarif642@gmail.com> <20251022220755.1026144-4-usamaarif642@gmail.com> <98bc658f-2ec6-43f5-a7e1-e9424450a850@intel.com> From: Usama Arif In-Reply-To: <98bc658f-2ec6-43f5-a7e1-e9424450a850@intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 23/10/2025 00:35, Dave Hansen wrote: > On 10/22/25 15:06, Usama Arif wrote: >> Instead, move _PAGE_BIT_NOPTISHADOW to use _PAGE_BIT_SOFTW1 (bit 9), > > Wait a sec, though... > > This isn't necessary once the previous 2 patches are applied, right? In kexec if the target kernels have patch 1 and 2, then this patch is not needed. Unfortunately, patches 1 and 2 are not livepatchable. Also backporting patches 1 and 2 to all previous kernels running in production in a large fleet is not very scalable. So if we want to run a kernel with 5 level pagetable in production (with the ability to kexec into a 4 level kernel that doesn't have the first 2 patches), then this patch would solve the problem. i.e. patches 1 and 2 solve the problem from the target kernels perspective, patch 3 solves it from the source kernel (if the target kernel doesnt have patches 1 and 2 applied). I mentioned this in the commit message as: " - The logistical challenge of patching all older kernels in production - The patch not being applicable for live patching " I can try and make the commit message clearer in the next revision.