Re: [Qemu-devel] [PATCH] target-s390x: fix LOAD MULTIPLE instruction on page boundary

[Aurelien Jarno <aurelien@aurel32.net>]

On 2015-05-25 22:47, Alexander Graf wrote:
> 
> 
> On 23.05.15 21:33, Richard Henderson wrote:
> > On 05/23/2015 12:59 AM, Aurelien Jarno wrote:
> >> On 2015-05-21 14:42, Richard Henderson wrote:
> >>> Hmm.  Seems to be un/under-specified in the PoO.  That said,
> >>
> >> There is a small sentence in the PoO, in chapter "Program Execution",
> >> section "Sequence of Storage Reference":
> >>
> >>    It can normally be assumed that the execution of
> >>    each instruction occurs as an indivisible event.
> > 
> > Ah, I didn't think to look in a different chapter.  ;-)
> > 
> >>> It would be nice to know if there ought to be similar up-front access
> >>> checking
> >>> for STM, to avoid errant partial stores.
> >>
> >> I have just checked, the same is also true for STM instructions, though
> >> it's probably more difficult to fix that in QEMU. Maybe we need a way to
> >> check if a load/store will succeed, preferably without using a helper.
> > 
> > I did just suggest a new helper in the "unaligned stores for mips r6"
> > thread. Therein we provide a probe_write helper that does assert that
> > the given page is writable, or raise the usual exception.  It leaves the
> > TLB updated, so a subsequent write should take the fast path.
> > 
> > It should be easy enough to extend that with an opcode so that we can
> > implement this for s390 as
> > 
> >     probe_write    addr + n * size - 1
> >     qemu_st        r0, addr
> >     qemu_st        r1, addr + 1*size
> >     ...
> > 
> > Hopefully for the edge case where both pages are unmapped, producing an
> > exception pointing to the last byte, rather than the first byte, is
> > acceptable.
> 
> So that means we should hold off on this patch for now as well and
> rather go for the probe approach?

For loads it's a bit different, but I guess we might come with a better
approach:

    load first word
    load last word
    save first word in the corresponding register
    save second word in the corresponding register
    load words in between and save them in the corresponding registers

So yes it might be a good idea to hold off this patch.

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net