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

From: Fenghua Yu <fenghua.yu@intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Fenghua Yu <fenghua.yu@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, H Peter Anvin <hpa@zytor.com>,
	Ashok Raj <ashok.raj@intel.com>, Alan Cox <alan@linux.intel.com>,
	Ravi V Shankar <ravi.v.shankar@intel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH 5/7] x86/vdso: Add vDSO functions for direct store instructions
Date: Mon, 23 Jul 2018 20:42:06 -0700	[thread overview]
Message-ID: <20180724034205.GA245659@romley-ivt3.sc.intel.com> (raw)
In-Reply-To: <10870309-993a-0abc-635a-92c05bbad140@kernel.org>

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