Re: [Qemu-devel] [PATCH] memory: Set notdirty_mem_ops validator

[Peter Xu <peterx@redhat.com>]

On Tue, Sep 03, 2019 at 05:50:56PM +0100, Peter Maydell wrote:
> On Tue, 3 Sep 2019 at 17:47, Tony Nguyen <tony.nguyen@bt.com> wrote:
> >
> > On Tue, Sep 03, 2019 at 11:25:28AM +0100, Peter Maydell wrote:
> > > On Mon, 2 Sep 2019 at 02:36, Tony Nguyen <tony.nguyen@bt.com> wrote:
> > > >
> > > > Existing read rejecting validator was mistakenly cleared.
> > > >
> > > > Reads dispatched to io_mem_notdirty then segfaults as there is no read
> > > > handler.
> > >
> > > Do you have the commit hash for where we introduced the
> > > bug that this is fixing?
> > >
> > > thanks
> > > -- PMM
> > >
> >
> > ad52878f97610757390148fe5d5b4cc5ad15c585.
> >
> > Please feel free to amend my commit message.
> 
> Thanks.
> 
> > I do not understand why sun4u booting Solaris 10 triggers the bug.
> 
> Do you have a backtrace of QEMU from the segfault? I'm having trouble
> thinking of what the situation is when we'd try to invoke the
> read handler on io_mem_notdirty...

I've no good understanding of how PHYS_SECTION_NOTDIRTY is used
yet... though from what I understand that's the thing this patch wants
to fix.  Because after the broken commit, this line will be
overwritten:

    .valid.accepts = notdirty_mem_accepts,

and accept() will be reset to NULL.

With that, memory_region_access_valid(is_write=false) could return
valid now (so a read could happen), while it should never, logically?

Regards,

-- 
Peter Xu