Re: [RFC PATCH v8 07/14] arm64/mm, xpfo: temporarily map dcache regions

[Tycho Andersen <tycho@tycho.ws>]

Hi,

On Wed, Feb 13, 2019 at 05:01:30PM -0700, Khalid Aziz wrote:
> From: Juerg Haefliger <juerg.haefliger@canonical.com>
> 
> If the page is unmapped by XPFO, a data cache flush results in a fatal
> page fault, so let's temporarily map the region, flush the cache, and then
> unmap it.
> 
> v6: actually flush in the face of xpfo, and temporarily map the underlying
>     memory so it can be flushed correctly
> 
> CC: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Juerg Haefliger <juerg.haefliger@canonical.com>
> Signed-off-by: Tycho Andersen <tycho@docker.com>
> ---
>  arch/arm64/mm/flush.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
> index 30695a868107..fad09aafd9d5 100644
> --- a/arch/arm64/mm/flush.c
> +++ b/arch/arm64/mm/flush.c
> @@ -20,6 +20,7 @@
>  #include <linux/export.h>
>  #include <linux/mm.h>
>  #include <linux/pagemap.h>
> +#include <linux/xpfo.h>
>  
>  #include <asm/cacheflush.h>
>  #include <asm/cache.h>
> @@ -28,9 +29,15 @@
>  void sync_icache_aliases(void *kaddr, unsigned long len)
>  {
>  	unsigned long addr = (unsigned long)kaddr;
> +	unsigned long num_pages = XPFO_NUM_PAGES(addr, len);
> +	void *mapping[num_pages];

What version does this build on? Presumably -Wvla will cause an error
here, but,

>  	if (icache_is_aliasing()) {
> +		xpfo_temp_map(kaddr, len, mapping,
> +			      sizeof(mapping[0]) * num_pages);
>  		__clean_dcache_area_pou(kaddr, len);

Here, we map the pages to some random address via xpfo_temp_map(),
then pass the *original* address (which may not have been mapped) to
__clean_dcache_area_pou(). So I think this whole approach is wrong.

If we want to do it this way, it may be that we need some
xpfo_map_contiguous() type thing, but since we're just going to flush
it anyway, that seems a little crazy. Maybe someone who knows more
about arm64 knows a better way?

Tycho