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Peter Anvin" , Breno Leitao Cc: Ingo Molnar , Dave Hansen , x86@kernel.org, Sean Christopherson , Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, vlad.wing@gmail.com, security@kernel.org, kernel-team@meta.com Subject: Re: [PATCH 6.18.y] x86/cpufeatures: Make X86_FEATURE leaf 17 Linux-specific In-Reply-To: <29B14106-19E4-4589-BE40-3D42BA646CE3@alien8.de> References: <20261111-stabe_srso-v1-1-45512320ba5e@debian.org> <20261112045703.GAavVHH8KA2yyCU7oc@fat_crate.local> <9492bce8-aab7-4982-8a0c-7d736c745e12@zytor.com> <29B14106-19E4-4589-BE40-3D42BA646CE3@alien8.de> Date: Fri, 13 Nov 2026 00:09:03 +0100 Message-ID: <87o6btsl4g.ffs@fw13> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain On Thu, Nov 12 2026 at 22:23, Borislav Petkov wrote: > On November 12, 2026 10:13:31 PM UTC, "H. Peter Anvin" wrote: >> Still, I have to wonder if it really makes sense to fold these leaves >> like this, since they are going to be populated soon. Surely we're not >> that short of bits? >> > > If you mean the leaves in scattered.c, I'm thinking the moment a leaf > appears there with half of its bits, it'll get moved to its own > separate leaf. Crap. Even if a new leaf has only _ONE_ bit defined then it's a new leaf. This whole "save space" nonsense has been source of a gazillion of subtle bugs in the past and we just continue to "fix" them instead of actually tackling the underlying problem. Linux specific leafs want to be in a space which is reserved by hardware for software purposes and not arbitrarily hacked into "holes". Hardware defined leafs have to be represented by a proper entry even if there is only one relevant bit in a leaf. That's what I have proposed long ago and the CPUID database project https://x86-cpuid.org/ laid the ground for changing this whole mess to something structured and well defined. Some initial changes have made their way into mainline, but then Intel stopped funding it and AMD has no money for that either... So this will trickle along for another two decades with the same bugs repeating themself over and over. WTF? Thanks, tglx