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

[Fenghua Yu <fenghua.yu@intel.com>]

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