linux-kernel - Re: [PATCH v12 6/7] x86/arch_prctl: Add ARCH_[GET|SET]

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Date:   Fri, 18 Nov 2016 09:49:34 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     Kyle Huey <me@...ehuey.com>,
        Robert O'Callahan <robert@...llahan.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Len Brown <len.brown@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        David Matlack <dmatlack@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        linux-kernel@...r.kernel.org,
        user-mode-linux-devel@...ts.sourceforge.net,
        user-mode-linux-user@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v12 6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID

On Fri, 18 Nov 2016, Ingo Molnar wrote:
> * Kyle Huey <me@...ehuey.com> wrote:
> > +	if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
> > +	    test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
> > +		set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
> > +	}
> > +
> 
> Why not cache the required MSR value in the task struct instead?
> 
> That would allow something much more obvious and much faster, like:
> 
> 	if (prev_p->thread.misc_features_val != next_p->thread.misc_features_val)
> 		wrmsrl(MSR_MISC_FEATURES_ENABLES, next_p->thread.misc_features_val);
> 
> (The TIF flag maintenance is still required to get into __switch_to_xtra().)
> 
> It would also be easy to extend without extra overhead, should any other feature 
> bit be added to the MSR in the future.

I doubt that. There are feature enable bits coming up which are not related
to tasks. So if we have switches enabling/disabling global features, then
we would be forced to chase all threads in order to update all misc_features
thread variables. Surely not what we want to do.

Thanks,

	tglx