[PATCH v5 8/8] arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol

[mark.rutland@arm.com (Mark Rutland)]

On Thu, Mar 19, 2015 at 07:30:03AM +0000, Ard Biesheuvel wrote:
> On 18 March 2015 at 21:24, Mark Rutland <mark.rutland@arm.com> wrote:
> >> >> >>>  ENTRY(stext)
> >> >> >>> +     adr_l   x8, boot_regs                   // record the contents of
> >> >> >>> +     stp     x0, x1, [x8]                    // x0 .. x3 at kernel entry
> >> >> >>> +     stp     x2, x3, [x8, #16]
> >> >> >>
> >> >> >> I think we should have a dc ivac here as we do for
> >> >> >> set_cpu_boot_mode_flag.
> >> >> >>
> >> >> >> That avoids a potential issue with boot_regs sharing a cacheline with
> >> >> >> data we write with the MMU on -- using __flush_dcache_area will result
> >> >> >> in a civac, so we could write back dirty data atop of the boot_regs if
> >> >> >> there were clean entries in the cache when we did the non-cacheable
> >> >> >> write.
> >> >> >>
> >> >> >
> >> >> > Hmm, I wondered about that.
> >> >> >
> >> >> > Could we instead just make it u64 __initconst boot_regs[] in setup.c ?
> >> >> >
> >> >>
> >> >> Never mind, it's easier just to do the invalidate right after, and I
> >> >> can drop the flush before the access.
> >> >
> >> > Yup.
> >> >
> >> > Annoyingly the minimum cache line size seems to be a word (given the
> >> > defnition of CTR.DminLine), which means you need a few dc ivac
> >> > instructions to be architecturally correct.
> >> >
> >>
> >> But that applies to cpu_boot_mode as well then?
> >
> > It writes a single word, so it happens to be safe.
> >
> >> I will add a call to __inval_cache_range() right after recording the
> >> initial values, that should do the right thing regarding llinesize
> >
> > That works, with one caveat: you'll need a dmb sy between the writes and
> > the call -- dc instructions by VA only hazard against normal cacheable
> > accesses, and __inval_cache_range assumes the caches are on and so
> > doesn't have a dmb prior to the dc instructions.
> >
> 
> Does it matter at all that __inval_cache_range() will mostly end up
> doing a civac for the whole array, since it uses civac not ivac for
> both non-cachelined aligned ends of the region, and the typical
> cacheline size is larger then the size of the array? Couldn't that
> also clobber what we just wrote with a stale cacheline?

Yes, though only if the memory were outside the footprint of the loaded
Image (which per the boot protocol should be clean to the PoC).

So I guess we should move the boot_regs structure back into head.S so it
doesn't fall outside 

Mark.