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

[Dave Hansen]

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?

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?