Re: [Qemu-devel] [PATCH] Add e500 instructions dcblc, dcbtls and dcbtstl as no-op

[Scott Wood <scottwood@freescale.com>]

On Fri, 1 Jul 2011 00:28:19 +0200
Alexander Graf <agraf@suse.de> wrote:

> 
> On 01.07.2011, at 00:23, Scott Wood wrote:
> 
> > On Fri, 1 Jul 2011 00:18:22 +0200
> > Alexander Graf <agraf@suse.de> wrote:
> > 
> >> 
> >> On 01.07.2011, at 00:11, Scott Wood wrote:
> >> 
> >>> Almost, but what if we have write permission but not read?
> >> 
> >> How would you write back data from a cache line when you haven't read it earlier?
> > 
> > The CPU can read it.  The program can't.
> 
> Hrm. We can always just call the check manually and trigger the respective interrupt :).

Yep.  A little trickier, but doable.

> >>> but what about a race with DMA from the I/O thread?
> >> 
> >> That'd simply be broken, but I don't quite see how it wouldn't with real hardware either :).
> > 
> > Real hardware doesn't generate a load/store sequence that the program didn't
> > ask for -- where's the breakage?
> 
> Real hardware flushes whatever contents are in that cache line to RAM, no? So it would collide with the DMA just as much. Or am I missing something?

If the DMA happens after the cache line is fetched, it'll be flushed,
whether locked or not.  But that's different from losing some of what the
device wrote.

-Scott