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

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Message-ID: <CALCETrX6oiYMvhsnttSFLuSSdOdG3+SyM0PQvBEVHHqZMXZGsQ@mail.gmail.com>
Date:   Mon, 23 Jul 2018 22:27:34 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 8:42 PM, Fenghua Yu <fenghua.yu@...el.com> wrote:
> 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

I think he meant that, if these helpers belong in the kernel at all,
then they belong in the vDSO.  But I think they mostly don't belong in
the kernel.

>
>> 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.

The underlying alternative infrastructure works in the vDSO.  You'd
need to introduce an alternate version of _static_cpu_has if
BUILD_VDSO that skips the boot_cpu_has fallback.