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

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Date:   Mon, 23 Jul 2018 18:48:00 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>
Cc:     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 07/23/2018 05:55 AM, Fenghua Yu wrote:
> User wants to query if direct store instructions are supported and use
> the instructions. The vDSO functions provides fast interface for user
> to query the support and use the instructions.
> 
> movdiri_supported and its alias __vdso_movdiri_supported check if
> movdiri instructions are supported.
> 
> movdir64b_supported and its alias __vdso_movdir64b_supported checks
> if movdir64b instruction is supported.
> 
> movdiri32 and its alias __vdso_movdiri32 provide user APIs for calling
> 32-bit movdiri instruction.
> 
> movdiri64 and its alias __vdso_movdiri64 provide user APIs for calling
> 64-bit movdiri instruction.
> 
> movdir64b and its alias __vdso_movdir64b provide user APIs to move
> 64-byte data through movdir64b instruction.
> 
> The instructions can be implemented in intrinsic functions in future
> GCC. But the vDSO interfaces are available to user without the
> intrinsic functions support in GCC and the APIs movdiri_supported and
> movdir64b_supported cannot be implemented as GCC functions.

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 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);

And all the VVAR stuff can be removed.