From: Thomas Gleixner <tglx@linutronix.de>
To: Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
Breno Leitao <leitao@debian.org>
Cc: Ingo Molnar <mingo@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
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
Date: Fri, 13 Nov 2026 00:09:03 +0100 [thread overview]
Message-ID: <87o6btsl4g.ffs@fw13> (raw)
In-Reply-To: <29B14106-19E4-4589-BE40-3D42BA646CE3@alien8.de>
On Thu, Nov 12 2026 at 22:23, Borislav Petkov wrote:
> On November 12, 2026 10:13:31 PM UTC, "H. Peter Anvin" <hpa@zytor.com> 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
next prev parent reply other threads:[~2026-11-12 23:09 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-11 16:06 [PATCH 6.18.y] x86/cpufeatures: Make X86_FEATURE leaf 17 Linux-specific Breno Leitao
2026-11-12 4:57 ` Borislav Petkov
2026-11-12 22:13 ` H. Peter Anvin
2026-11-12 22:23 ` Borislav Petkov
2026-11-12 23:09 ` Thomas Gleixner [this message]
2026-11-12 23:33 ` Borislav Petkov
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