[RFC PATCH 0/2] arm64: change PoC D-cache flush to PoU

[catalin.marinas@arm.com (Catalin Marinas)]

On Mon, Dec 14, 2015 at 08:46:03AM -0800, Ashok Kumar wrote:
> On Mon, Dec 14, 2015 at 03:11:18PM +0000, Catalin Marinas wrote:
> > On Mon, Dec 14, 2015 at 05:27:42AM -0800, Ashok Kumar wrote:
> > > Also deferred dcache flush in __cpu_copy_user_page to __sync_icache_dcache.
> > > May I know why I/D sync is needed in __cpu_copy_user_page? My understanding 
> > > is that any self modifying code in userspace is supposed to take care of the 
> > > coherency using the respective cache flush system call. 
> > 
> > I think it may have been there for historical reasons on arch/arm
> > (115b22474eb1 - "ARM: 5794/1: Flush the D-cache during
> > copy_user_highpage()") and imported in arch/arm64. But looking back at
> > this, I don't think we need it for two reasons: dynamic linker no longer
> > relocating symbols in a text page (and cause CoW) and set_pte_at()
> > already calling __sync_icache_dcache().
> 
> In that case, shall we get rid of the flush in __sync_icache_dcache also
> by not calling flush_dcache_page in __cpu_copy_user_page?

We should still keep it. Assuming that we get CoW mostly on no-exec
pages, the penalty should be minimal since set_pte_at() only cares about
executable mappings. But I can't guarantee that user space doesn't do
any CoW on executable mappings. We could for example have some JIT'ed
code that forked and the child changes some literal pool data (not
instructions) causing a CoW. It would not expect the I/D caches to
become incoherent.

-- 
Catalin