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

From: [email-masked] (Steve Capper)
Date: Tue, 11 Feb 2014 08:44:03 +0000

On Mon, Feb 10, 2014 at 05:12:00PM +0000, Russell King - ARM Linux wrote:
> On Mon, Feb 10, 2014 at 04:51:16PM +0000, Steve Capper wrote:
> > 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.
> 
> Great, so we can get rid of that and the get_page() on the vdso data
> page below.
> 
> > > > +
> > > > +	/* 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?
> 
> My point is that we can already view this page directly by dereferencing
> vdso_start - do we really need to perform this apparant test of the MMU?
> If the MMU isn't working in this way, we have much bigger and more
> fundamental problems...
> 

I see, yes I think people would notice the MMU not working :-).
This code also tests the alignment of vdso_start in a roundabout way.
I'm not sure whether or not an explicit alignment check to PAGE_SIZE
would be beneficial instead of the test mapping.

Cheers,
-- 
Steve