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Subject: Re: [PATCH v10 6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID
To: Kyle Huey <me@kylehuey.com>,
        "Robert O'Callahan" <robert@ocallahan.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        x86@kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Shuah Khan <shuah@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Borislav Petkov <bp@suse.de>, Peter Zijlstra <peterz@infradead.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Len Brown <len.brown@intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        David Matlack <dmatlack@google.com>
References: <20161108183956.4521-1-khuey@kylehuey.com>
 <20161108183956.4521-7-khuey@kylehuey.com>
 <30f2ec3e-d0c8-8dd2-837f-3380237d843c@zytor.com>
Cc: linux-kernel@vger.kernel.org,
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        user-mode-linux-user@lists.sourceforge.net,
        linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        kvm@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <6de65e0f-73d4-4e6b-1945-f69036423806@zytor.com>
Date: Tue, 14 Mar 2017 12:17:06 -0700
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On 03/14/17 12:01, H. Peter Anvin wrote:
> On 11/08/16 10:39, Kyle Huey 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));
>> +	}
>> +
>>  	if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
>>  	    test_tsk_thread_flag(next_p, TIF_NOTSC)) {
>>  		/* prev and next are different */
>>  		if (test_tsk_thread_flag(next_p, TIF_NOTSC))
>>  			hard_disable_TSC();
>>  		else
>>  			hard_enable_TSC();
>>  	}
> 
> I'm unhappy about this part: we already do two XORs on these after bit
> extraction, which is quite inefficient; and at least theoretically we
> could be indirecting though the ->stack pointer for every one if gcc
> can't tell it won't have changed (we really need to get thread_info
> moved into the task_struct allocation and away from the kernel stack,
> especially since on x86 the pointer is the same size as the vestigial
> structure it points to.)
> 

Nevermind, I was accidentally looking at v10 not v15 of this patchset.
My bad.

	-hpa