From: Sean Christopherson <seanjc@google.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@intel.com>, X86 ML <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/cpu: Start documenting what the X86_FEATURE_ flag testing macros do
Date: Fri, 20 Jan 2023 00:35:02 +0000 [thread overview]
Message-ID: <Y8nhtjFcsB63UsmQ@google.com> (raw)
In-Reply-To: <Y8kRuUwsKxYsk1AX@zn.tnic>
On Thu, Jan 19, 2023, Borislav Petkov wrote:
> Another belated reply... ;-\
>
> On Thu, Nov 10, 2022 at 11:27:08PM +0000, Sean Christopherson wrote:
> > What about doing the opposite and folding cpu_feature_enabled()'s build-time
> > functionality into static_cpu_has() _and_ boot_cpu_has(), and then dropping
> > cpu_feature_enabled()? That way the tradeoffs of using the static variant are
> > still captured in code (cpu_feature_enabled() sounds too innocuous to my ears),
> > and as an added bonus even slow paths benefit from build-time disabling of features.
> >
> > Hiding the use of alternatives in cpu_feature_enabled() seems like it will lead to
> > unnecessary code patching.
>
> Actually, tglx and I have a sekrit plan - a small preview below. I don't have
> answers to replacing all functionality we have yet but it is a good start and
> the goal is to eventually get rid of all the gunk that has grown over the years.
> +struct func_1 {
> + /* EDX */
> + union {
> + struct {
> + u32 fpu : 1, vme : 1, de : 1, pse : 1,
> + tsc : 1, msr : 1, pae : 1, mce : 1,
> +
> + cx8 : 1, apic : 1, __rsv2 : 1, sep : 1,
> + mtrr : 1, pge : 1, mca : 1, cmov : 1,
> +
> + pat : 1, pse36 : 1, psn : 1, clfsh : 1,
> + __rsv3 : 1, ds : 1, acpi : 1, mmx : 1,
> +
> + fxsr : 1, sse : 1, sse2 : 1, ss : 1,
> + htt : 1, tm : 1, __rsv4 : 1, pbe : 1;
> + };
> + u32 edx;
> + } __packed;
> +};
IMO, switching to bitfields would be a big step backwards. Visually auditing the
code is difficult, e.g. when reviewing brand new leafs, and using cpufeatures.h as
a quick reference is essentially impossible.
E.g. I often look at cpufeatures.h when I want to know the leaf+bit of a feature,
because trying to find the same info in the SDM or APM is often painful.
prev parent reply other threads:[~2023-01-20 0:35 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-07 21:15 [PATCH] x86/cpu: Start documenting what the X86_FEATURE_ flag testing macros do Borislav Petkov
2022-11-07 22:13 ` Dave Hansen
2022-11-08 9:42 ` Borislav Petkov
2022-11-10 23:27 ` Sean Christopherson
2023-01-19 9:47 ` Borislav Petkov
2023-01-20 0:35 ` Sean Christopherson [this message]
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