Subject: [RFC/PATCH v2] ARM: vDSO gettimeofday using generic timer architecture

From: [email-masked] (Steve Capper)
Date: Mon, 10 Feb 2014 16:51:16 +0000

Hi Russell,

On Sun, Feb 09, 2014 at 10:20:23AM +0000, Russell King - ARM Linux wrote:
> On Fri, Feb 07, 2014 at 05:05:49PM -0600, Nathan Lynch wrote:
> > +	/* Grab the vDSO code pages. */
> > +	for (i = 0; i < vdso_pages; i++) {
> > +		pg = virt_to_page(&vdso_start + i*PAGE_SIZE);
> > +		ClearPageReserved(pg);
> > +		get_page(pg);
> > +		vdso_pagelist[i] = pg;
> > +	}
> 
> Why do we want to clear the reserved status?  This looks over complicated
> to me.
> 

This looks like it was inherited from the PowerPC code where the
behaviour of set_pte_at would change dependent on whether or not the
page was reserved (set_pte_at->set_pte_filter->maybe_pte_to_page). I
think we can safely remove this from ARM and ARM64.

> > +
> > +	/* Sanity check the shared object header. */
> > +	vbase = vmap(vdso_pagelist, 1, 0, PAGE_KERNEL);
> > +	if (vbase == NULL) {
> > +		pr_err("Failed to map vDSO pagelist!\n");
> > +		return -ENOMEM;
> > +	} else if (memcmp(vbase, "\177ELF", 4)) {
> > +		pr_err("vDSO is not a valid ELF object!\n");
> > +		ret = -EINVAL;
> > +		goto unmap;
> > +	}
> 
> Why do we need to vmap() pages which are already accessible - vdso_start
> must be part of the kernel image, and therefore will be accessible via
> standard mappings.
> 

This is a dress rehersal for install_special_mapping more than anything.
If we map the page, and look at the first 4 bytes, are they what we
expect?

> > +
> > +	/* Grab the vDSO data page. */
> > +	pg = virt_to_page(vdso_data);
> > +	get_page(pg);
> > +	vdso_pagelist[i] = pg;
> 
> Same goes for this.
> 

We need the data page so we can remap it into user space via
install_special_mapping.

Cheers,
-- 
Steve