Re: [RFC, PATCH 19/22] x86/mm: Implement free_encrypt_page()

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Mar 05, 2018 at 11:07:16AM -0800, Dave Hansen wrote:
> On 03/05/2018 08:26 AM, Kirill A. Shutemov wrote:
> > +void free_encrypt_page(struct page *page, int keyid, unsigned int order)
> > +{
> > +	int i;
> > +	void *v;
> > +
> > +	for (i = 0; i < (1 << order); i++) {
> > +		v = kmap_atomic_keyid(page, keyid + i);
> > +		/* See comment in prep_encrypt_page() */
> > +		clflush_cache_range(v, PAGE_SIZE);
> > +		kunmap_atomic(v);
> > +	}
> > +}
> 
> Have you measured how slow this is?

No, I have not.

> It's an optimization, but can we find a way to only do this dance when
> we *actually* change the keyid?  Right now, we're doing mapping at alloc
> and free, clflushing at free and zeroing at alloc.  Let's say somebody does:
> 
> 	ptr = malloc(PAGE_SIZE);
> 	*ptr = foo;
> 	free(ptr);
> 
> 	ptr = malloc(PAGE_SIZE);
> 	*ptr = bar;
> 	free(ptr);
> 
> And let's say ptr is in encrypted memory and that we actually munmap()
> at free().  We can theoretically skip the clflush, right?

Yes we can. Theoretically. We would need to find a way to keep KeyID
around after the page is removed from rmap. That's not so trivial as far
as I can see.

I will look into optimization after I'll got functionality in place.

-- 
 Kirill A. Shutemov

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