From: Paolo Bonzini <pbonzini@redhat.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Nadav Amit <namit@cs.technion.ac.il>,
Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
the arch/x86 maintainers <x86@kernel.org>,
kvm <kvm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [RESEND PATCH 1/3] x86: Adding structs to reflect cpuid fields
Date: Fri, 19 Sep 2014 15:40:29 +0200 [thread overview]
Message-ID: <541C324D.2080608@redhat.com> (raw)
In-Reply-To: <20140919075814.GA30491@nazgul.tnic>
Il 19/09/2014 09:58, Borislav Petkov ha scritto:
>> > The trivial example is feature bits like XSAVE. We query them all the
>> > time without checking the family when they were first introduced,
>> > don't we?
> The feature bits would obviously be 0 if features are not supported.
And similarly, Intel would not extend a bit from 16 to 17 bits if it
weren't zero on all older processors.
> However, even there
>
> "16 - Reserved - Do not count on the value."
>
> I'm quoting Intel's CPUID doc 241618-037 from 2011 (there might be a
> newer one though), the CPUID(1).ECX description.
Once that bit gets a meaning in newer processors, the same meaning will
work retroactively for existing processors. That's just how CPUID is
used. Nobody checks families before testing bits, Intel/AMD do not even
suggest that.
> Do you have a guarantee that this won't happen in the future and break
> all your fancy bitfields assumptions?
No guarantee, but were that to happen, I'd expect tar and feathers
spectacles around Intel's engineering headquarters.
Paolo
next prev parent reply other threads:[~2014-09-19 13:41 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-16 12:22 [PATCH 0/3] x86: structs for cpuid info in x86 Nadav Amit
2014-09-16 12:22 ` [PATCH 1/3] x86: Adding structs to reflect cpuid fields Nadav Amit
2014-09-16 12:22 ` [PATCH 2/3] x86: Use new cpuid structs in cpuid functions Nadav Amit
2014-09-16 12:22 ` [PATCH 3/3] KVM: x86: Using cpuid structs in KVM Nadav Amit
2014-09-16 13:22 ` [PATCH 0/3] x86: structs for cpuid info in x86 Ingo Molnar
2014-09-16 20:19 ` Nadav Amit
2014-09-17 12:37 ` Ingo Molnar
2014-09-17 12:45 ` Borislav Petkov
2014-09-17 12:54 ` [RESEND PATCH " Nadav Amit
2014-09-17 12:54 ` [RESEND PATCH 1/3] x86: Adding structs to reflect cpuid fields Nadav Amit
2014-09-17 13:21 ` Borislav Petkov
2014-09-17 13:53 ` Nadav Amit
2014-09-17 14:06 ` Borislav Petkov
2014-09-17 15:04 ` Radim Krčmář
2014-09-17 15:22 ` Borislav Petkov
2014-09-18 0:29 ` Radim Krčmář
2014-09-18 7:19 ` Borislav Petkov
2014-09-18 10:00 ` Radim Krčmář
2014-09-18 13:06 ` Paolo Bonzini
2014-09-18 13:26 ` Borislav Petkov
2014-09-18 13:36 ` Paolo Bonzini
2014-09-19 7:58 ` Borislav Petkov
2014-09-19 8:59 ` Nadav Amit
2014-09-19 10:32 ` Borislav Petkov
2014-09-19 13:40 ` Paolo Bonzini [this message]
2014-09-19 14:44 ` Borislav Petkov
2014-09-17 14:10 ` Peter Zijlstra
2014-09-17 12:54 ` [RESEND PATCH 2/3] x86: Use new cpuid structs in cpuid functions Nadav Amit
2014-09-17 12:54 ` [RESEND PATCH 3/3] KVM: x86: Using cpuid structs in KVM Nadav Amit
2014-09-17 14:12 ` [PATCH 0/3] x86: structs for cpuid info in x86 Peter Zijlstra
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