linux-kernel - Re: [PATCH 5/7] x86/vdso: Add vDSO functions for direct store instructions

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Date:   Mon, 23 Jul 2018 20:42:06 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH 5/7] x86/vdso: Add vDSO functions for direct store
 instructions

On Mon, Jul 23, 2018 at 06:48:00PM -0700, Andy Lutomirski wrote:
> On 07/23/2018 05:55 AM, Fenghua Yu wrote:
> >The instructions can be implemented in intrinsic functions in future
> >GCC. But the vDSO interfaces are available to user without the
> I'm not convinced that any of this belongs in the vDSO at all.  You could
> just add AT_HWCAP (or AT_HWCAP2) flags for the new instructions. Or user

Thomas asked to use vDSO. Please see the discussion thread:
https://lkml.org/lkml/2018/6/19/316

> code could use CPUID just like for any other new instruction. But, if there
> really is some compelling reason to add this to the vDSO, then see below:
> >+notrace bool __vdso_movdiri_supported(void)
> >+{
> >+	return _vdso_funcs_data->movdiri_supported;
> return static_cpu_has(X86_FEATURE_MOVDIRI);

But boot_cpu_data (used in static_cpu_has) cannot be accessed by user
unless mapped in VVAR. So this change cannot be compiled.

> And all the VVAR stuff can be removed.

The VVAR stuff needs to map the kernel data _vdso_funcs_data to user
space before user can access it.

Thanks.

-Fenghua