Re: [PATCH, RFC] virtio_blk: add cache control support

[Rusty Russell <rusty@rustcorp.com.au>]

On Thu, 24 Mar 2011 10:54:05 +0100, Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> Am 24.03.2011 04:05, schrieb Anthony Liguori:
> >> ie. lguest and S/390 don't trap writes to config space.
> >>
> >> Or perhaps they should?  But we should be explicit about needing it...
> > I don't think we ever operated on the assumption that config space writes would trap.
> > 
> > I don't think adding it is the right thing either because you can do byte access to the config space which makes atomicity difficult.
> 
> There is the additional problem, that s390 has no MMIO and,therefore,
> there is no real HW support for trapping writes to an area. You can
> use page faults, or read-only faults on newer systems, but this is 
> expensive. In addition, page faults only deliver the page frame, but
> not the offset within a page.

That's not *really* a problem, since you have control over the
config_set operation and could do whatever you wanted.

But I wanted to make sure we're all on the same page: you *can't* rely
on the host knowing immediately what you write to the config space.  If
you want that, an actual queued request is necessary...

Thanks,
Rusty.