Re: [PATCH 4/9] iommu/amd: Simplify pagetable freeing

[Robin Murphy]

On 2021-12-06 12:40, Joerg Roedel wrote:
> On Tue, Nov 23, 2021 at 02:10:39PM +0000, Robin Murphy wrote:
> > For reasons unclear, pagetable freeing is an effectively recursive
> > method implemented via an elaborate system of templated functions that
> > turns out to account for 25% of the object file size. Implementing it
> > using regular straightforward recursion makes the code simpler, and
> > seems like a good thing to do before we work on it further. As part of
> > that, also fix the types to avoid all the needless casting back and
> > forth which just gets in the way.
>
> Nice cleanup! The stack of functions came from the fact that recursion
> was pretty much discouraged in the kernel. But in this case it looks
> well bounded and should be fine.

I did wonder about explicitly clamping lvl to ensure that it couldn't 
possibly recurse any further than the multi-function version, but given 
that you'd need to craft a suitable bogus pagetable in addition to 
corrupting the arguments to be able to exploit it at all, that seemed 
perhaps a little too paranoid. Happy to add something like:

	if (WARN_ON(lvl > PAGE_MODE_7_LEVEL))
		return NULL;

if you like, though.

> > +static struct page *free_pt_lvl(u64 *pt, struct page *freelist, int lvl)
> > +{
> > +	u64 *p;
> > +	int i;
> > +
> > +	for (i = 0; i < 512; ++i) {
> > +		/* PTE present? */
> > +		if (!IOMMU_PTE_PRESENT(pt[i]))
> > +			continue;
> > +
> > +		/* Large PTE? */
> > +		if (PM_PTE_LEVEL(pt[i]) == 0 ||
> > +		    PM_PTE_LEVEL(pt[i]) == 7)
> > +			continue;
> > +
> > +		p = IOMMU_PTE_PAGE(pt[i]);
> > +		if (lvl > 2)
>
> I thinkt this function deserves a couple of comments. It took me a while
> to make sense of the 'lvl > 2' comparision. I think it is right, but if
> I have think again I'd appreciate a comment :)

Heh, it's merely a direct transformation of the logic encoded in the 
existing "DEFINE_FREE_PT_FN(...)" cases - I assume that's just an 
optimisation, so I'll add a comment to that effect.

Thanks,
Robin.